Condor Cycles, WC1X 8PP
A bike shop on Gray's Inn Road.
They sell a range of rather good bikes, particularly road bikes and single-speed/fixed-gear bikes, as well as components and accessories.
The staff are quite impressively knowledgeable: when doop went to buy new shoes, the chap in the shoe section took one look at his feet and knew the correct size; he was similarly adept at gauging saddle sizes.
Condor are an excellent place to go if you would like a mostly custom bike built up from scratch, and they have a fitting jig to ensure that you go for the correct size of frame.