I have to put my agent to do the dirty work and find the best offers in town. Never been in London but I quess the shops aren´t at the same street...
Oddly they sort of are. Specialist shops seem to "cluster" as is now happening in Helsinki it seems with partioaita, the North Face shop and that other one all being next to each other and Camu not so far away. So if you have an hour on Kenisington High Street, and another around Covent Garden you'll see more than outdoor kit than a man could possibly need.
Having said that, a much better idea would be to go climbing in either North Wales or the Lake District and then do some after climb shopping in Llanberis or Ambleside where there are just loads of gear shops. Ambleside has to be the English Chamonix (although minus the glaciers obviously).
One other idea, if you have a friend living in London you could order online and have delivered to them. www.needlesports.com
is maybe the best, Rock and Run are very good as well but Needlesports do free delivery in the UK normally making them a wee bit cheaper.