Before you start

An unlocked phone (most phones sold in the UK and EU are; check if yours came from a carrier abroad)
Nothing else for PAYG — no ID, no proof of address, no registration is required to buy or activate a SIM
For a monthly contract only: a UK bank account for direct debit and usually a UK credit history (a hurdle for newcomers)

Step-by-step

1
Pick a PAYG SIM (or eSIM) to start
Order a free PAYG SIM online from an MVNO like Giffgaff or Smarty before you arrive, or buy one in any supermarket, phone shop or airport kiosk. Many networks also offer an eSIM you can download and activate instantly from your phone — handy before your physical SIM arrives.
OnlineWho: YouSame day; posted SIMs arrive in a few daysSIM usually free or about £1; eSIM free to obtain
2
Activate and top up — no registration needed
Insert the SIM (or install the eSIM), follow the activation steps, and add a 'bundle' or 'goodybag' of data, minutes and texts. There is no ID check or registration: you simply pay for a bundle, typically renewed monthly with no contract tying you in.
Mobile appWho: YouMinutesPAYG bundles roughly £6–20/mo depending on data — verify current plans
3
Use Giffgaff or Smarty while you build credit history
These MVNOs are the standard newcomer choice: cheap, no credit check, 30-day rolling plans you can cancel any time, and they run on the big networks (Giffgaff on O2, Smarty on Three). They let you avoid the contract credit check entirely until you have a UK footprint.
Mobile appWho: YouOngoingFrom around £6–10/mo for a typical data bundle — verify current plans
4
Optional: move to a monthly contract later
Once you have a UK bank account and some credit history, a 12–24 month contract from EE, Vodafone, O2 or Three can bundle a handset and more data. Contracts require a direct debit and a credit check, so they are usually a few months in, not a day-one move. You can keep your number when you switch (text 'PAC' to 65075).
In personWho: YouOnce you have a bank account + credit historySIM-only ~£10–25/mo; handset contracts more — verify current plans

Documents you’ll need

An unlocked phone (or a UK-bought handset)
A payment card to buy top-up bundles (PAYG)
For a contract only: UK bank account details for direct debit and ID for the credit check
Your existing number's PAC code if you want to keep a UK number when switching providers

Things most newcomers don’t know

No SIM registration — a real contrast with many countries.

Unlike much of the world, the UK does not require ID or registration to buy and activate a SIM. You can land, grab a PAYG SIM at the airport or a supermarket, and have a working UK number the same hour — no paperwork.

Source: Ofcom — pay-as-you-go mobile guidance

Giffgaff and Smarty are the newcomer default for a reason.

Monthly contracts need a UK bank account and a credit check you cannot pass on arrival. These MVNOs run on the big networks but skip the credit check, use cheap 30-day rolling plans, and let you cancel any time — ideal until you have a UK footprint.

Source: Giffgaff / Smarty plan pages

An eSIM gets you connected before a physical SIM arrives.

Several networks and MVNOs offer eSIMs you can buy and activate instantly from your phone, so you need not wait for a posted SIM or find a shop — useful in your first hours in the country.

Source: Carrier eSIM pages (EE/Vodafone/O2/Three)

Check your phone is unlocked before you rely on a UK SIM.

A handset bought on contract abroad may be network-locked and reject a UK SIM. Confirm it is unlocked (or ask your old carrier to unlock it) before you arrive, or you may be stuck buying a new phone.

Source: Ofcom — locked and unlocked mobiles

Common mistakes to avoid

Signing up for a contract on day one and failing the credit check — start on PAYG instead
Bringing a network-locked handset from abroad that won't accept a UK SIM
Letting a PAYG SIM sit unused for months until it is deactivated
Losing your UK number when switching providers by not requesting a PAC code first