I can recommend all three shops mentioned just above. Red Dot is quite nice--I've browsed there several times, although I haven't bought much from them. But they're friendly folks with a good reputation.
Aperture is fantastic, too. Reasonable prices, good selection, nice shop (the tea and banana bread are excellent, lol) and it's convenient to the Photographer's Gallery, the British Museum, etc., so definitely a nice Saturday trip into the city.
And if you don't mind venturing all the way down to the southern edge of London, Croydon Photo Centre (better known locally as High Street Radio) is my usual hangout. Possibly the best prices of all, the owner (Reg Roach) is a wonderful old fellow with a real wealth of knowledge he doesn't mind sharing, and he'll cut you a deal that will make both of you happy. I've bought tons of stuff from him, some Leica but other things as well. It's one of those increasingly rare specialist shops, family-run for decades and catering to the eccentric who's really dedicated to analogue photography.
They don't have nearly as much Leica stock as the first two places, though, so you might want to look at the website (
www.croydonphotocentre.co.uk) first or call down to see if they have anything that interests you before making the journey. Reg currently has a very nice M6 and a good user M2 available, as well as a pristine MP (with 0.85 finder) which I had a look at today. And I know he keeps some other things stashed away as well, which he will bring out to tempt you. He's open Tuesdays, Thursdays, Fridays and Saturdays from 10.30 to 5.
If you can time it right for when Red Dot is open on a Saturday, you could hit all three. I'd make the trip all the way down to Croydon first, if you're coming that far, to catch Reg when he opens up. Then take a fast train from East Croydon to Victoria if you're hitting Aperture first, or London Bridge (and thence to Old Street via the Northern Line) if you're going to Red Dot. Whichever you visit first, the number 19 or 38 bus will get you to the other in about 10 minutes.
