This gets asked quite often, so I'll write this up now and copy and paste it in future!
These are all camera stores I've been to personally in London. Some I bought from, some I didn't.
Silverprint (near Waterloo Station) - Bought film and developing equipment, excellent service. Awful stock system, so phone ahead.
LCE (Strand) - Visited with a colleague, bought a 55mm Macro in good condition. They did say they'd buy his 18-135 off him but reneged two weeks after, which left a bit of a bitter taste.
Aperture UK (Museum Street, near Holborn) - Fantastic. Bought lenses, a light meter and other bits and bobs. Mainly
second hand, some new stuff. Mainly Nikon, but they have a good selection of other gear.
Camera City (Little Russell Street, near Holborn) - Seems alright, never bought anything there. I pop in when I'm around to see if they have a bargain.
Jacobs (New Oxford Street) - Bought a lens for my RZ67. There was a chip on the lens (reflected in the price) but they let me shoot a roll with it and held the lens for a day while I got it developed. They've got a surprising range of
second hand gear for all brands, from 35mm to large format.
Microglobe (Galen Square, near Holborn) - Don't bother. Grey import, shoddy service, no cheaper than legit good online. Been in but never bought anything.
York Cameras (Bury Place, near Holborn) - Great. Good advice, good stock selection (mainly Canon but some Nikon and others). Bought bits and bobs from them.
Camera World (Wells Street, off Oxford Street) - Great. Bought a few bits and bobs and a few lenses off them.
Grays of Westmister (Pimlico really

) - Got a couple of lenses from them, excellent service.
That's all the ones I can think of that I've bought from/had dealings with!
When I feel like shopping around for second hand gear I tend to visit Jacobs, Aperture and Camera City. I've got a few bargains that way (stuff that I wouldn't have bought if I hadn't seen it mind...)