40GB SIM only deals UK September 2026: ranked by 12-month cash

Last updated: Friday 4 September 2026 (Europe/London).
Next update: when iD’s 40GB 12-month £7 JSON-LD offer moves, when VOXI’s Unltd Social 40GB £8 card stops selling, when giffgaff’s £8 Data Boost ends on 4 December 2026, when SMARTY or spusu reprint a cheaper exact-40GB shop price, or when the live SIM hub shows a cheaper same-allowance 12-month total on a retailer page I can open.
By: Switch Editorial Team, Switch Squid Ltd

I am Switch. I compare SIMs for a living, and 40GB SIM only deals UK September 2026 is the mid-tier rung comparison sites keep answering with a “30GB+” filter that quietly dumps 35GB, 45GB, 50GB and 60GB onto the same grid.

uSwitch has no dedicated 40GB guide URL — /mobiles/guides/40gb-sim-only-deals/ 404s. You land on the SIM-only compare table, tick a box, and hope the card still means 40GB by checkout. MoneySuperMarket’s SIM hub was still wearing a stale “Best of June 2026” label when I last checked the public page. This URL is one tier: exact 40GB, priced from retailer shops on Friday 4 September 2026.

Our SIM only hub is every network and every allowance, ranked by total monthly cost. This page stays on 40GB. If you are still shopping the rung below, use today’s neighbour guide: 30GB SIM only deals UK September 2026.

The cash lead this morning is iD Mobile 40GB on a 12-month plan at £7.00 on idmobile.co.uk/sim-only-deals — twelve months £84.00, unlimited UK minutes and texts, Three masts, no annual price rises printed on the JSON-LD offer. One pound a month behind that flat rate is VOXI Unltd Social 40GB at £8 on voxi.co.uk/plans (social extras on Vodafone) and iD’s own 40GB 1-month at £8 if you refuse to lock a year. giffgaff’s promotional £8 for 40GB on 18 months (Data Boost window to 4 December 2026) matches that £96 year on O2 masts if you will take the longer commitment.

Ofcom’s Communications Market Report 2026 puts average monthly data per mobile data user at 11.8GB in 2025 (CMR 2026 PDF). So 40GB is a heavy-user / commute-streamer / hotspot bucket — more than triple the average. If last month was under 12GB, start with 20GB or 30GB. If it was 80GB-plus, use 100GB or unlimited.

I will not invent a 40GB SKU. I will not rank a uSwitch tile the retailer’s own site does not sell. Every pound in the ranking table below is dated to a retailer page I am standing behind on 4 September 2026.

Filter 40GB on the hub

Open the SIM only hub. Set data around 40GB, pick 30-day or 12-month, sort by total monthly cost. Commission does not move a row. If a 50GB or 60GB card is sitting on top of a “40GB” filter, you filtered wrong — or the live feed mixed a neighbour tier.

Need a new handset instead of airtime? That is a different bill: compare phone deals or phone deals hub.

Already sure you want a thinner or fatter cap? Neighbour guides are 30GB SIM only deals UK, 20GB SIM only deals UK, 100GB SIM only deals UK and cheapest unlimited SIM only UK 2026. This URL stays on 40GB.

What I verified today

Seven retailer-linked exact-40GB plans (plus one longer-lock twin). Dated Friday 4 September 2026. Near-tier cards (30GB, 50GB, 60GB) live in the notes, not as fake 40GB rows. Comparison exclusives that are not on the shop today live in “What I dropped”.

iD Mobile SIM-Only 40GB 12 Months, idmobile.co.uk/sim-only-deals (JSON-LD Product offer on the page), fetched 4 September 2026: £7.00 a month, 12-month term, 40GB, unlimited minutes and texts, no annual price rises, eSIM available on the journey, Three network. Twelve-month total £84.00, average £7.00. This is the cash lead for exact 40GB.

iD Mobile SIM-Only 40GB 24 Months, same shop JSON-LD, fetched 4 September 2026: also £7.00 a month. Same year cash as the 12-month card. I am not ranking it above the 12-month twin — locking twenty-four months for the same pound is a worse trade unless you already know you will stay.

iD Mobile SIM-Only 40GB 1 Month, same shop, fetched 4 September 2026: £8.00 a month rolling. Twelve-month total £96.00. The flexibility tax is £12 a year versus locking the 12-month £7 card.

VOXI Unltd Social 40GB plan 8, voxi.co.uk/plans, fetched 4 September 2026: product display name with priceGrossOVR £8.00, dataValue 40, marked sellable (sku family fortyUnlimitedSocial). Vodafone masts. Unlimited UK minutes and texts plus the social-unlimited extras that plan name implies. Twelve-month total £96.00, average £8.00. Different product from a plain 40GB all-data SIM — I am ranking the cash, and I am saying the extras are why someone might still pick it over iD’s rolling £8.

giffgaff £8 18-month plan with Data Boost to 40GB, documented on giffgaff.com/terms/promotions (offer window from 24 August 2026 to 4 December 2026), O2 network, unlimited UK calls and texts, EU roam fair-use commonly 5GB on giffgaff copy. Shop HTML was bot-walled from this machine; I am ranking the promotions-terms pound, not inventing a goodybag. Twelve months at £8 is £96.00. After the 18-month boost period, standard allowance on that £8 rung reverts unless you change plan — read the promotions page before you treat 40GB as permanent.

spusu 40, spusu.co.uk/tariffs / plans JSON-LD, fetched 4 September 2026: £9.90 a month rolling. 40GB, unlimited minutes, unlimited texts. EU roam 10GB / 500 minutes / 500 SMS. Three masts. Twelve-month total £118.80, average £9.90.

SMARTY 40GB, smarty.co.uk/plans/40gb-data, fetched 4 September 2026: £10.00 a month, 1-month rolling, unlimited UK calls and texts, no annual price rises, no credit check, eSIM, EU roam with a 12GB fair-use limit on the plan page, Three network. Twelve-month total £120.00, average £10.00.

What I dropped

Comparison hubs and thin “40GB” round-ups still try to sell these as 40GB. Retailer shops do not match, or I could not verify the pound on the shop today.

• O2 first-party 40GB at £8 — widely listed on uSwitch / comparison tiles with April rises. o2.co.uk shop HTML from this crawl did not print a clean matching 40GB pay-monthly SKU I could stand behind (PAYG cards showed other rungs). Comparison exclusive → dropped. Rank the shop when O2 prints it.

• Talkmobile Flex 40GB — does not exist. talkmobile.co.uk/sim-only-deals Flex ladder jumps 30GB at £6.95 → 60GB at £7.95. The 60GB card is a neighbour, not a 40GB row.

• Lebara 40GB — lebara.co.uk HTML this morning did not show a clean 40GB pay-monthly SKU. Dropped until the shop prints it.

• Three first-party 40GB — three.co.uk often hits queue walls from this machine. I will not invent a Three pound. iD and SMARTY / spusu already cover Three masts with verified shop prices.

If a tile is cheaper than iD’s shop £7 and the retailer’s own site does not print that exact 40GB SKU, the tile is not a deal. It is a trap for people who do not click through.

Ranked by what you actually pay

Four columns. Ongoing rate is what the bill is while the current promo or rolling rate holds. Average is the 12-month total divided by 12. Every row below is retailer-verified (or promotions-terms verified for giffgaff) on 4 September 2026.

£7.00 is the iD 12-month month. £84.00 is that year. £8.00 is the VOXI / iD 1-month / giffgaff month. £96.00 is that year.

Over twelve months iD’s 12-month 40GB is £84. The £8 rolling cluster (VOXI social, iD 1-month, giffgaff boost) lands at £96 — £12 more for the year. spusu is £118.80. SMARTY is £120. The gap from iD’s lead to SMARTY is £36 a year for the same labelled allowance — enough to fund a decent case and a screen protector, then some.

Phone under purple light — 40GB SIM only deals ranked by 12-month cash.

£7 for 40GB is the year to beat

If Three coverage works at your postcodes and you will stay twelve months, the cheapest retailer-verified exact-40GB year I can print today is iD Mobile’s 12-month 40GB at £7 × 12 = £84 on idmobile.co.uk. No April rise on that SKU’s page copy. eSIM is on the journey if your handset wants it.

If you want month-to-month flexibility on Three, iD’s 40GB 1-month at £8 is £96 for the year — twelve pounds more than locking. That is the honest flexibility tax at this tier.

If Vodafone coverage is the mast that works, VOXI’s Unltd Social 40GB at £8 is the verified rolling card — with social extras baked into the plan name. Do not buy it only because a filter said “40GB” and ignore the extras; buy it because you will use them, or because Vodafone signal is the point.

If O2 coverage is the point and you will accept eighteen months, giffgaff’s promotional Data Boost on the £8 plan is the O2-network mid-tier story until 4 December 2026. Reopen giffgaff.com before you pay — my shop crawl was blocked, and boosts revert after the contract window unless you change plan.

spusu at £9.90 is the rolling Three option with a fatter EU roam data line (10GB) if that matters more than the monthly pound. SMARTY at £10 is the Three rolling card with the cleanest “no credit check / no annual rise / cancel anytime” brochure — you pay for that clarity.

40GB is well above average. Check Settings first.

Ofcom’s 2026 CMR figure is 11.8GB average per mobile data user in 2025. A 40GB SIM is more than triple that average. This URL is for people who stream on the commute, hotspot a laptop some evenings, share maps all day on site, or want headroom after a 30GB month that kept brushing the ceiling.

Illustrative only, labelled as such: at a rough 3GB per hour for HD streaming, 40GB is about thirteen hours of video a month before maps, social apps and updates eat the rest. If Settings → Mobile Data (iPhone) or the Android data-usage screen showed 25–35GB last month with a week still to go, you are in the right shop. If it showed 10–18GB, you are overpaying for headroom — start on the 30GB guide instead. If it showed 55GB+, stop reading mid-tier pages and open 100GB or unlimited.

Desk setup for checking mobile data use on a 40GB plan.

I am not going to pretend 40GB is “unlimited in disguise”. Heavy 4K, multi-device hotspotting, or a week abroad under a tight roam cap will still bite. That is why the 100GB and unlimited pages exist.

Same £7 buys 30GB rolling or 40GB locked on iD

This is the September-specific trap at the mid-tier — and it is why this guide sits next to today’s 30GB page.

iD Mobile’s exact 30GB 1-month card is £7. iD’s 40GB 12-month is also £7 on idmobile.co.uk today. Same network. Same monthly pound. Ten more gigabytes if you will commit twelve months.

If you want rolling flexibility (moving house, testing coverage, waiting on a handset unlock), take the 30GB 1-month on the neighbour guide — or pay £8 for iD’s 40GB 1-month on this page. If you already know Three works and you will stay a year, the 40GB 12-month is the better cash-per-gigabyte card. Do not lock a year for the smaller allowance out of habit.

Talkmobile’s nearest Flex cards are 30GB at £6.95 and 60GB at £7.95 — there is no 40GB Talkmobile SKU to “meet in the middle”. SMARTY’s nearest card is this page’s 40GB at £10. Do not let a comparison filter labelled “40GB+” sell you a 50GB or 60GB bill without saying so.

You are buying a mast, not a logo

40GB on a dead mast is 0GB. The four UK networks still sit under the MVNO names.

Coverage checkers lie less than marketing pages. Test the mast that already works on a friend’s phone in your kitchen before you port.

30-day vs 12-month vs 18-month at this tier

iD’s 1-month 40GB, VOXI, spusu and SMARTY are short commitment. That is the point of mid-tier shopping in September: you can leave after a month if the mast is wrong.

12-month locks only win when the monthly pound drops enough to beat rolling cash and you will actually stay. At exact 40GB this morning, that story is clear: iD’s £7 on 12 months beats every verified rolling 40GB card I can print by at least £12 a year.

18-month giffgaff is a different trade. You get O2 masts and a promotional 40GB boost at £8 — but you are tied longer, and the boost is not a permanent rewrite of the £8 goodybag. Read the promotions page end date (4 December 2026 to join) and the post-boost allowance before you celebrate.

April rises still exist on many big-network SIMs. iD’s page copy on the 40GB plans says no annual price rises. SMARTY says the same. VOXI / Vodafone-family and O2-family cards can still sit under group rise policies — read the checkout small print, not the comparison tile.

Roaming and eSIM, without the brochure

SMARTY prints EU roam with a 12GB fair-use limit on the 40GB plan page. spusu 40 lists 10GB EU roam data plus 500 minutes / 500 SMS. giffgaff’s EU inclusive data on these promos is commonly 5GB regardless of the UK allowance — a 40GB UK plan does not mean 40GB in Spain. iD’s pay-monthly roam destination lists and fair-use caps sit on the plan journey — confirm the cap before you treat holiday data like home data. VOXI / Vodafone roam rules sit on checkout.

eSIM: SMARTY and iD both lean into eSIM on the plan pages. spusu supports eSIM on the UK journey. giffgaff is heavily eSIM-capable for members who want it. If your phone is eSIM-only, check the retailer basket before you celebrate the £7 lead.

How I ranked these (and what I refuse to do)

I open the retailer’s own SIM page. I take the monthly pound and the contract length printed there. I multiply by twelve. I sort by that 12-month cash. I do not sort by commission. I do not sort by “from” price on a comparison exclusive.

I refuse to:

• count a 35GB, 45GB, 50GB or 60GB tile as 40GB because a filter said “40GB+”

• print an O2 / Talkmobile / Lebara pound I could not match on the shop today

• hide social extras (VOXI) or boost revert clauses (giffgaff) behind a single “40GB” label

• pretend cashback portals change the retailer sticker price

Affiliate links, when we use them, do not move a row in this table. If iD is cheaper on idmobile.co.uk than a partner deep link, the shop price wins the ranking.

FAQ

What is the cheapest 40GB SIM only deal in the UK right now?
On retailer pages I opened on 4 September 2026, iD Mobile 40GB on a 12-month plan at £7.00 a month (£84.00 over twelve months) is the cash lead for unlimited UK calls and texts. VOXI Unltd Social 40GB at £8 and iD’s 40GB 1-month at £8 are the leading rolling options at £96 for the year.

Is 40GB enough data?
For heavy mid-tier users, yes — Ofcom’s 2025 average was about 11.8GB per mobile data user. 40GB suits regular video on the commute, maps all day, and light-to-medium hotspotting. Check last month’s usage in Settings before you buy. Under ~20GB, look at 30GB. Over ~60GB, look at 100GB or unlimited.

Should I take iD 30GB or iD 40GB?
If you want month-to-month flexibility at £7, take 30GB at £7 on 1-month (neighbour guide). If you will stay twelve months, iD’s 40GB 12-month is also £7 today — same cash, more data. Do not lock a year for the smaller allowance out of habit.

Why did you drop O2 and Talkmobile?
Because I could not verify an exact 40GB shop SKU on o2.co.uk this morning (comparison tiles do not count), and Talkmobile’s Flex ladder has no 40GB rung (30GB then 60GB). Comparison teasers that are not on the retailer shop do not get a row.

Is giffgaff’s 40GB permanent?
No. The Data Boost on the £8 18-month plan is a promotional allowance for customers who join in the offer window (to 4 December 2026 on the promotions terms). After the boost period, standard allowance on that rung can revert. Read giffgaff.com/terms/promotions before you treat 40GB as forever.

Do these prices rise every April?
iD’s 40GB plan copy and SMARTY’s 40GB page say no annual price rises. Other networks still use April rises on many SIMs. Read the plan’s own rise clause at checkout — not a blog table from last spring.

Can I keep my number?
Yes. Text to Switch (or the older PAC/STAC path where it still applies) moves the number. Start the port from the new retailer’s app or checkout after the SIM is active.

Where do I compare live SIMs across every allowance?
Use the SwitchSquid SIM only hub — filter around 40GB, sort by total monthly cost, then click through to the retailer before you pay.

Compare live 40GB SIMs

Retailer pages move. This ranking is dated 4 September 2026. Before you pay, open the shop URL in the table, confirm the pound, and check coverage at your postcodes.

Ready to filter the live grid? Compare SIM only deals on SwitchSquid — every network, sorted by what you actually pay, not by who shouted loudest on a comparison tile.