Last updated: 5 November 2026
Next update: when Smarty or O2 Unlimited prices move, or when iD Mobile’s 25GB 30-day plan changes price.
By: Switch Editorial Team, Switch Squid Ltd
Sources: Smarty Unlimited plan page; O2 SIM-only deals page; iD Mobile 25GB plan.
Prices as listed on 9 October 2026: O2 Classic Unlimited was £23 a month, Smarty Unlimited £20 a month. If you don’t need true Unlimited, iD Mobile 25GB was £6 a month on a 30-day rolling plan in the iD Mobile basket opened from this page on 4 November 2026.
O2 update: on 1 November 2026, o2.co.uk listed Classic Unlimited at £23 a month for 24 months as a limited-time price, so check O2’s page for the price on the day you order. On 4 November 2026, O2’s order page listed a 10GB Mini 30-day plan at £7 a month as an online exclusive — see our O2 30-day SIM guide.
As of 9 October 2026, the cheapest true unlimited SIMs on this page are Smarty Unlimited at £20 a month (30-day rolling, Three) and O2 Classic Unlimited at £23 a month (24 months, a limited-time price O2 listed on 1 November 2026). With the Smarty deal button on this page, Smarty Unlimited was £17 a month, as listed on Smarty’s site on 4 November 2026. If 25GB is enough, iD Mobile’s 25GB plan was £6 a month on a 30-day rolling plan in the iD Mobile basket opened from this page on 4 November 2026. On 1 November 2026, giffgaff 40GB was £8 a month and giffgaff 25GB £10 a month, both on 18-month plans, and giffgaff says the 40GB for £8 plan is for sign-ups before 4 December 2026. Confirm the price on the retailer’s page before you join.
Network: giffgaff runs on O2’s network, not EE’s.
iD Mobile 25GB — £6/mo · 30-day rolling
- 25GB of 5G data, unlimited UK calls & texts
- 30-day rolling plan at £6 a month in the iD Mobile basket opened from this page, as listed on 4 November 2026 — no long contract
- Runs on Three’s network — confirm the £6 price in the iD Mobile basket before you pay
Cheapest unlimited and budget SIMs compared
Plan | £/mo | Term | Network | Status |
|---|---|---|---|---|
Smarty Unlimited | £20 | 30-day rolling | Three | On smarty.co.uk |
O2 Classic Unlimited | £23 | 24 months | O2 | Limited-time price listed on o2.co.uk on 1 November 2026 |
iD Mobile 25GB | £6 | 30-day rolling | Three (iD Mobile) | In the iD Mobile basket from this page, 4 November 2026 |
Smarty 32GB rolling | £8 | 30-day | Three | Mid-data alternative, listed 1 November |
Smarty 5GB rolling | £6 | 30-day | Three | Light-data alternative, listed 1 November |
Two unlimited options: Smarty Unlimited, 30-day rolling, at £17 a month when you open Smarty from this page (£20 on smarty.co.uk), and O2 Classic Unlimited at £23 a month for 24 months, a limited-time price O2 listed on 1 November 2026.
Smarty Unlimited — £17/mo · 1-month rolling
- Unlimited data with unlimited UK calls & texts on Three’s network, as listed on Smarty’s site on 4 November 2026
- £17 a month when you open Smarty from this page (£20 on Smarty’s own site), and you can leave any month
- Main limitation: 12GB EU roaming fair use. Check Three’s coverage at your postcode and confirm £17 on Smarty’s page before you pay
Unlimited plans come with fair-use and roaming terms that differ by provider, so check the inclusive EU roaming allowance on the plan page before you travel.
How to choose an unlimited or big-data SIM
Take Smarty Unlimited £20 rolling if: Three coverage works for you, you want true Unlimited without a long lock, and you are happy buying from smarty.co.uk.
Take O2 Classic Unlimited £23 / 24 months if: you want Unlimited on O2’s network and are happy with a 24-month plan. O2 listed this as a limited-time price on 1 November 2026, so check O2’s page for the price on the day you order. It rises £2.50 each April.
Take iD Mobile 25GB if: you don’t need Unlimited at all. 25GB at £6 a month on a 30-day rolling plan (Three network), as listed in the iD Mobile basket from this page on 4 November 2026, is the cheapest pick on this page.
Skip Unlimited if: you only need 40–100GB (Smarty’s mid-size plans or giffgaff’s 40GB plan may cost less), or you need Three-mast coverage instead of O2.
Prefer flexible on Three? Smarty rolling SIMs
If you only need a mid-size data bucket, Smarty’s 30-day rolling plans on Three stay cancel-anytime, no credit check, no mid-contract price rise. The prices below were listed on SMARTY’s site on 1 November 2026.
| Plan | £/mo | Data | Term | Network |
|---|---|---|---|---|
| Smarty 32GB | £8 | 32GB | 30-day | Three |
| Smarty 5GB | £6 | 5GB | 30-day | Three |
| Smarty Unlimited | £20 | Unlimited | 30-day | Three |
True unlimited on Smarty is £20 a month on smarty.co.uk/plans/unlimited. On 5 October 2026 it listed no speed cap, a 12GB EU roaming fair-use allowance and UK tethering. If you don’t need Unlimited, iD Mobile’s 25GB plan at £6 on a 30-day rolling contract is the cheaper choice.
More Smarty context: Smarty SIM deals · Smarty guide
Who should skip Unlimited
If you only need 40–100GB, start with our cheapest unlimited and big-data SIM guide and the 100GB SIM guide rather than paying for true unlimited. For giffgaff’s own range, see our giffgaff Unlimited guide.
What to do next
Decide whether you still need true Unlimited, or whether 25–40GB is enough.
For Unlimited on Three: open Smarty Unlimited with the deal button on this page (£17 a month on 1 November 2026, or £20 on smarty.co.uk).
For Unlimited on O2: check O2’s page for the Classic Unlimited price on the day you order (O2 listed £23 for 24 months as a limited-time price on 1 November 2026).
For the cheapest option: iD Mobile 25GB £6 (30-day rolling, Three network).
Port with a PAC if you are keeping your number; do not cancel the old line early.
Check the monthly price after any introductory period before you commit.
Keep your number (PAC)
Request a PAC from your current provider and port into your new network — do not cancel early or you risk losing the number and any remaining credit. Guide: how to get a PAC code.
Coverage, FUP, EU roam
giffgaff rides O2’s UK radio network. Smarty rides Three. Check coverage for home, commute and the places you hotspot before you celebrate unlimited maths — use the Ofcom mobile coverage checker. Indoor basement flats and thick-walled stock still need a signal check.
Unlimited plans remain subject to fair-use and acceptable-use policies. Heavy permanent tethering of a home broadband substitute is the classic way to attract scrutiny on any UK MVNO unlimited tier. For travel, check each retailer’s roaming footnotes on the day you buy.
Smarty 100GB — £10/mo · 1-month rolling
- 100GB of 5G and 4G data with unlimited UK calls & texts on Three’s network, as listed on Smarty’s site on 4 November 2026
- £10 a month when you open Smarty from this page (£12 at Smarty’s standard price), no credit check and no annual price rises
- Main limitation: Three’s network, not O2. Check coverage at your postcode first, then confirm £10 on Smarty’s page
Sources + affiliate disclosure
Prices: Smarty Unlimited £20 as listed on smarty.co.uk on 9 October 2026. O2 Classic Unlimited £23 and Classic 100GB £20, both for 24 months, were limited-time prices listed on o2.co.uk on 1 November 2026. Smarty Unlimited £17 and Smarty 100GB £10 were the prices shown when opening Smarty from this page, as listed on Smarty’s site on 4 November 2026. iD Mobile 25GB was £6 a month on a 30-day rolling plan in the iD Mobile basket opened from this page, as listed on 4 November 2026.
More guides: SIM only deals · cheapest unlimited SIM UK · Smarty SIM deals · O2 30-day SIM guide.
Affiliate disclosure: SwitchSquid may earn a commission when you buy through the deal buttons on this page. It doesn’t change the retailer’s price.
Always confirm the price and any activation date in the retailer’s checkout or app.
FAQ
What is this guide about?
It compares the cheapest unlimited SIMs on sale as of 9 October 2026, Smarty Unlimited at £20 and O2 Classic Unlimited at £23, with a budget pick, iD Mobile 25GB at £6 on a 30-day rolling plan (as listed on 4 November 2026). Always confirm the price and terms on the retailer page before you buy or switch.
When was this last updated?
Smarty’s standard price on this page is as listed on 9 October 2026, O2’s and giffgaff’s prices as listed on 1 November 2026, and the Smarty deal-button and iD Mobile prices as listed on 4 November 2026. Prices can change, so confirm them on the retailer’s page on the day you act.
What are the cheapest unlimited SIMs in this guide?
Smarty Unlimited was £17 a month on a 30-day rolling plan (Three) with the deal button on this page as listed on 4 November 2026, or £20 on smarty.co.uk. O2 Classic Unlimited was £23 a month for 24 months, a limited-time price O2 listed on 1 November 2026, so check O2’s page for the price on the day you order. If you don’t need Unlimited, iD Mobile 25GB at £6 on a 30-day rolling plan is cheaper.
What data plans does giffgaff sell?
On 1 November 2026, giffgaff 40GB was £8 a month on an 18-month plan, and its 25GB plan was £10 a month on the same term. giffgaff says the 40GB for £8 plan is for sign-ups before 4 December 2026. It runs on O2’s network. Confirm the price on giffgaff.com before you pay.
Can I keep my mobile number?
Yes on most UK SIMs. Text PAC to 65075 from your current phone, then start the port during the new provider’s join flow so activation lands when you expect.
Do I need a credit check?
Rolling SIMs often skip a hard credit check; 12–24 month contracts usually include one. Check the provider’s join page for the path you pick.
Do you earn money from the deal buttons?
Yes. SwitchSquid may earn a commission when you buy through the deal buttons, and it doesn’t change the price you pay. Confirm the plan name, data, term and fees in the retailer’s basket before you pay.
What if the price looks different at checkout?
Prices and personalised offers can change. If checkout doesn’t match the guide, refresh the retailer page or leave the basket rather than paying a price you didn’t expect.
