i don't live in london, but have been a couple of times on holiday, and checked out as many guitar shops as i could... :lol: I imagine denmark street is probably the best plan if you only have one day- there are better shops in the surrounding area, but if you only have one day you'd likely only manage to make it to one shop (because of all the travelling involved), so it's probably a better idea to go to denmark street where you have several shops right next to each other...
if you head to denmark street, you've got the several shops on denmark street, plus another couple on charing cross road, plus ivor maraints just off the near end (near relative to denmark street) of oxford street. When I was there last, there was still sound control under the Virgin Megastore at the near end of oxford street and the soho soundhouse on charing cross road right next to denmark street, but SC went bust (and SC owned soho soundhouse too, i think), I'm not sure what happened to those two stores as I haven't been to London since.
EDIT: as I said, I'm not local to London or anything, so there may be other, better, lesser-known shops of which I'm not aware. :) Depends on what type of stuff you're after too- I thought that Denmark street was pretty darn expensive (I wouldn't really want to buy anything there in most of the shops- it didn't help that one of the shops quoted me double the RRP for an effects pedal I was trying :x ), but had a really good selection of guitars and effects pedals- but the amp selection wasn't great. So if you're mainly planning on trying amps, it might be worth going further afield, even if it does mean you can only make it to one shop.
