Family SIM sharing UK October 2026: multi-line maths

Last updated: 5 November 2026
Next update: when giffgaff, Smarty or iD Mobile change the price of their family-friendly plans.
By: Switch Editorial Team, Switch Squid Ltd

“Family SIM sharing” in the UK usually means several individual SIMs on fair mid-data plans rather than one pooled bucket, and a practical anchor is giffgaff’s 40GB 18-month plan at £8 a month on O2, with iD Mobile’s 15GB at £6 a month on a 1-month plan for lighter users, or iD Mobile’s 50GB at £8 a month on a 12-month plan where Three’s signal is better, as listed on 28 October 2026. On 1 November 2026, giffgaff’s 18-month plans were 40GB for £8 a month and 25GB for £10 a month, and giffgaff says the 40GB for £8 plan is for sign-ups before 4 December 2026. For heavier users, our giffgaff alternatives guide compares bigger plans. This guide sizes each line in a household, rather than ranking Unlimited brands. We may earn a commission if you buy using the deal buttons on this page.

Sources: giffgaff, iD Mobile, Mozillion, Smarty and O2 plan pages, with prices as listed between 9 October and 4 November 2026.

giffgaff 40GB — £8 / 18 months

£8 a month on an 18-month minimum term, as listed on giffgaff.com on 4 November 2026 for sign-ups before 4 December 2026 — subject to giffgaff’s terms
40GB UK data + unlimited UK calls & texts on O2 radio
Main limitation: 18-month commitment and early-exit charges if you leave early
Solid mid-data fit for one adult line in a multi-SIM household

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Offer facts (prices as listed between 9 and 28 October 2026)

Plan	Term	Promo price	Data	Catch
giffgaff 40GB	18-month contract	£8/mo	40GB + unlimited UK calls/texts	Commitment; early exit charge if you leave early
Smarty Unlimited	30-day rolling	£20/mo	Unlimited on Three	Only for the person who regularly goes beyond 80GB
iD Mobile 15GB	1-month plan	£6/mo	15GB on Three	Light user; leave at the end of any month
iD Mobile 50GB	12-month contract	£8/mo	50GB on Three	12-month commitment
Mozillion 80GB	1-month plan	£9.90/mo	80GB on EE	For the household’s heavy user

giffgaff runs on O2; Smarty and iD Mobile run on Three; Mozillion runs on EE. Coverage differs by postcode — run each retailer’s checker for home, school-run and workplace before you commit a whole household.

Parent and two teenagers comparing their phone plans at a kitchen island

Most value SIMs are individual accounts. True multi-line “share a data pool” products exist on some big networks, but the cheapest options for many households in late 2026 are still several small SIMs sized to each person’s usage. That is the approach in this guide: size each line to the person who uses it.

Hotspotting on a mid-data plan empties the bucket fast. If one person tethers a laptop daily, size that line up (or put them on Unlimited) rather than hoping family Wi-Fi will always save them.

When giffgaff 40GB £8 wins for a family line

Choose the 40GB 18-month plan when O2 coverage is fine, the person uses roughly 20–40GB most months, and you are willing to keep the line for the term. At £8 it costs far less than Unlimited while leaving headroom for maps, social video and school apps. Pair it with a lighter plan, such as iD Mobile 15GB at £6, for a teen who mostly uses home Wi-Fi. If Three’s signal is better at home, iD Mobile’s 50GB is the same £8 a month on a 12-month plan.

Line on Three? iD Mobile 50GB for £8 a month, 12-month plan

50GB of 5G data with unlimited UK calls and texts for £8 a month on a 12-month plan, as listed in the iD Mobile basket opened from this page on 4 November 2026
10GB more than giffgaff’s 40GB for the same £8, on a 12-month term instead of 18 months
Main limitation: runs on Three’s network, not O2, so check Three coverage at home, school and work first

Choose this plan

When rolling mid-data or Unlimited fits better

Rolling mid-data (Smarty £8 or iD Mobile 15GB £6): students leaving for uni, short-term housemates, or anyone who may cancel within months.
Bigger 80GB plan (Mozillion 80GB £9.90 a month, 1-month plan): the household hotspotter or heavy commuter who regularly uses 40–80GB.
Unlimited (Smarty £20 rolling or O2 Classic Unlimited on a 24-month plan): only the person who regularly goes beyond 80GB.

Our separate rolling-vs-18-month and giffgaff-vs-Smarty Unlimited guides cover term and brand fights in more depth. This URL stays on multi-line household sizing.

Fit summary

Good for giffgaff 40GB £8: stable household members with solid O2 signal and mid-data habits.
Good for rolling mid-data: lines that may leave or pause soon.
Good for Unlimited: only the people who actually need it — not every adult “just in case”.

Important conditions

18-month plans carry early-termination charges — read the contract summary.
giffgaff says the 40GB for £8 plan is for sign-ups before 4 December 2026. Check the retailer page when you order.
Fair-use, roaming caps and member eligibility sit in each retailer’s terms.
eSIM vs physical SIM: order early if a join window is tight.
Keep PAC texts (65075) ready if you are porting numbers line by line.

Switching steps for a household

1) List every phone and estimate monthly data from Settings.
2) Check O2 (giffgaff) and Three (Smarty) coverage for home, school and work.
3) Assign mid-data vs Unlimited per person — do not clone Unlimited for everyone.
4) Order the primary mid-data line through the giffgaff 40GB £8 deal button if that is the fit.
5) Port numbers after each new SIM is active.
6) Diary contract ends so you are not surprised by post-benefit prices.

Final decision card

Stable mid-data adults on O2 → giffgaff 40GB £8 / 18 months.
Need exit doors → rolling mid-data per person.
Stable mid-data adults on Three → iD Mobile 50GB £8 / 12 months.
One heavy user → Mozillion 80GB £9.90 a month on a 1-month plan, or Unlimited on that line only; keep others mid-data.

One heavy user? Mozillion 80GB for £9.90 a month, no long contract

80GB with unlimited UK calls and texts on EE’s network, £9.90 a month on a 1-month plan, as listed on Mozillion’s site on 4 November 2026
The first month is paid when you order; Mozillion lists no credit check, no mid-contract price rises and EU roaming in 41 countries
Main limitation: runs on EE’s network, so check EE coverage at that person’s home and work. Confirm 80GB and £9.90 on Mozillion’s page before you pay

Check availability

Household examples

Two adults and a light-use teen: giffgaff 40GB at £8 a month for each adult and iD Mobile 15GB at £6 a month (listed on 28 October 2026) for the teen, rather than three Unlimited plans.

One remote worker hotspotting: Mozillion 80GB at £9.90 or Unlimited on the worker; 40GB or 15GB on everyone else.

Student leaving in January: prefer rolling mid-data for that line even if parents stay on 18-month plans.

For term-length Unlimited maths see rolling vs 18-month October. For brand Unlimited detail see giffgaff vs Smarty Unlimited. For Smarty-only pricing see the Smarty SIM only guide. This URL is the family multi-line twin.

Parental controls, bill-payer names and PAC hygiene

Multi-line households fail on paperwork as often as on coverage. Decide who is the bill payer on each account, whether a teen’s line sits under a parent login, and how top-ups or Direct Debits will be funded. giffgaff’s member model is individual — each plan is its own account even when parents pay. That is fine for clarity; it is painful if nobody diaries the renewal texts.

Porting four numbers in one evening is a recipe for locked phones and half-ported PAC codes. Sequence the moves: activate the new SIM, confirm data works on Wi-Fi off, then text PAC to 65075 and complete the port. Keep the old SIM until the number lands. If a line is on a handset still network-locked, unlock first — a perfect £8 plan will not save a locked phone.

Roaming, EU caps and school-trip reality

Family SIMs get stress-tested on holidays. Unlimited UK data is not unlimited roaming. Mid-data plans burn faster when hotel Wi-Fi fails and everyone maps offline areas. Read each retailer’s roaming allowance before you promise a “free Europe” half-term. For a two-week trip, a temporary travel eSIM can be cheaper than upsizing every home line to Unlimited “just in case”.

School devices on mobile data are another silent bucket drain. If a tablet without Wi-Fi sits on a 20GB plan, expect the allowance to vanish during coursework week. Prefer Wi-Fi homework rules, or put that device on a line sized for it — do not assume the adult Unlimited plan automatically covers every gadget in the house unless the retailer terms say hotspotting is included without a separate cap.

Credit checks and 18-month commitment across several lines

Taking out three or four 18-month contracts raises the exit risk if someone leaves home, emigrates or changes name on the bill. Rolling mid-data on the uncertain lines and 18-month mid-data on the stable lines is often the calmer household design. Credit checks are more common on longer commitments — read checkout prompts rather than assuming “SIM-only means no check” for every adult in the family.

FAQ

Can a UK family share one SIM data allowance?

Most value SIMs are individual, so each person needs their own plan. Estimate each person’s data use, then choose a plan for each line. Some major networks sell multi-line data pools, so compare those separately if you need a true shared allowance.

Is giffgaff 40GB £8 a good family plan?

Yes, for mid-data users who can keep an 18-month term and get good O2 coverage at home, school and work. Heavy streamers or anyone who hotspots a laptop may still need Unlimited.

How much is giffgaff 40GB a month?

giffgaff listed 40GB at £8 a month on an 18-month plan on 4 November 2026, for sign-ups before 4 December 2026. Check the plan terms on giffgaff.com, because add-ons and roaming are charged separately.

Should every family member buy Unlimited?

Usually not. Size each line to how much data that person really uses, and keep Unlimited for the heaviest user. Giving everyone Unlimited is often an expensive habit rather than a need.

Is Smarty better for families?

Smarty’s month-to-month plans can suit someone who may cancel soon, or a household where Three coverage is stronger than O2. iD Mobile also runs on Three, with 50GB for £8 a month on a 12-month plan as listed on 28 October 2026. Confirm the price on each retailer’s page before you order.

Can we keep our numbers?

Yes. Text PAC to 65075 from each current phone to get a switching code. Give the code to the new provider when you order, and your number moves across, usually the next working day.

Do you earn commission?

Yes. SwitchSquid may earn a commission at no extra cost to you when you buy through the deal buttons on this page. It does not change the price you pay.