the Cyclesurgery at Spitalfields (not the one in the market, the other one) is MTB-specific and worth a browse if you’re near Liverpool Street.
And as much as I hate to admit it, the Evans in Spitalfields Market is pretty good and has a big big room downstairs with loads of stuff in.
Condor is a nice roadie-oriented shop with lots of bits to look at and clothes.
On Your Bike at London Bridge is big and browseable.
Problem nowadays is that most of the London bikes shops cater for commuters so are full of hybrids, faux fixies and every possible item of clothing available in hi-viz yellow.
EDIT: I really want to go to Brick Lane Bikes but the people in there make me fell all punchy.