London: world-class signal, and the one UK city where the local SIM wins

The price argument, first

Most of our city pages end up recommending a travel eSIM. London is the case where the local SIM genuinely undercuts it. UK SIM cards require no ID at all, they are sold in every Tesco and Sainsbury's around the clock, and giffgaff prices sit at £10 for 6GB or £25 for unlimited data.

Against that, the cheapest verified travel eSIM rows for the UK — read from the providers' own sites on 13 August 2026 — are Airalo's Uki Mobile at $4.00 for 1GB over three days and Saily at $4.49 for 1GB over seven days. Scale up and the pattern holds: Saily's 10GB for 30 days is $18.99 and Uki Mobile's is $19.00. (Airalo sells a second UK package, Uki Mobile+, dearer at every size — $5.00 for that 1GB and $23.50 for that 10GB month.) Converted at any recent exchange rate, the supermarket SIM is cheaper per gigabyte.

So the real decision is about time, not money. An eSIM installed at home means you land at Heathrow already connected. A local SIM means twenty minutes in a shop and a UK phone number, for a few pounds less. For a weekend, take the eSIM. For two weeks or more, walk into the Tesco.

Signal, above ground and below

London's coverage is as good as anywhere on earth. All four UK carriers deliver excellent 4G and 5G across central London, EE leads on 5G, and the averages here are 53 Mbps on 4G and 200 Mbps on 5G. EE is the pick if you have a choice — widest 5G, and the best penetration into deep buildings, which matters more in a city of thick Victorian masonry than the headline speed does.

The Underground has 4G at stations on most central lines, and increasingly in the tunnels themselves. It is not universal. The deeper Jubilee line tunnels north of Canary Wharf drop out, as do sections of the Circle and District lines. Above ground, the interiors of the Tower of London and Westminster Abbey can be weak, along with concrete-heavy older buildings generally.

None of this requires action beyond a habit: check your route before you go through the barriers, not after.

Arriving at Heathrow

Heathrow is well set up for this. Sim Local sells eSIMs on EE at Terminals 2, 3 and 5, with vending machines running 24 hours. Boots and WHSmith sell physical SIMs in all terminals.

The fastest arrival, though, involves buying nothing at the airport: install an eSIM before you fly and activate it on the Elizabeth line into Paddington. You are connected before you reach the city, and you have skipped the arrivals queue entirely. Airport SIMs run about $10, which is not a rip-off by international standards, but it is still more than the supermarket.

The apps that make London easier

Citymapper is the definitive London navigation app and is meaningfully better than Google Maps for multi-modal routing across Tube, bus, rail and walking. TfL's own app covers Oyster and contactless. Deliveroo and Uber Eats work everywhere central, and Santander Cycles is the bike hire scheme.

One thing worth understanding about London transport: you need data to plan a journey, but not to pay for one. Payment is contactless card or phone at the gate, and it works whether or not you have signal. So a dead phone battery is a bigger risk than a dead data plan.

Dates that flatten the network

Trooping the Colour in June and the State Opening of Parliament both create extreme crowding around Westminster, and congestion with it. Notting Hill Carnival on the August bank holiday draws around two million people and brings severe network congestion across the carnival area.

On those days, treat data as unreliable in the affected postcodes regardless of what you bought. Agree meeting points in advance, and screenshot anything you will need to show at a gate.

Verdict

EE 5G is dominant in central London, so any EE-based eSIM will give you 5G at the major sights. Buy an eSIM for a short trip, buy a giffgaff SIM from a supermarket for a long one, and do not buy anything at the airport unless you have to. The full provider-by-provider numbers are on the UK hub.