Most mainline railway stations and many Underground (subway) stations have photobooths
You don't seriously expect anyone to list the hundreds or thousands of photo booths, do you, especially as you don't specify where in London (it's quite a big place, in case you didn't know)! As others have said, most stations (railway, Tube, airport) have them, as do many post offices and supermarkets, Boots chemists, department stores, shopping centres, etc.
Mainline railway stations will have them, and very often post offices so people can get their passport photos done. My local shopping centre has a couple scattered around.
Most train stations have photo booths
Thousands and thousands of them, rail stations, hypermarkets, bus stations
