Only place I can think of is Oxford guitar center - But I've NEVER been there.
If you're going to northern london, check out Electrohill, not the biggest stock, but a lovely amp tech runs the place.
Denmark Street has the main attractions, Rose Morris has AMAZING gear but HORRENDOUS prices. World of Music is full of everything that Rockers doesn't have. Rockers is your place for Mesa, Ibanez, ESP.
Soundcontrol is pretty epic if you've never been there - There's a very supermarket sense, so if you're not used to playing in guitar shops it may be more to your type of thing.