A wide variety of products, styles and types of riding, i.e., I've learnt to avoid boutique "commuter" shops like the plague from bitter experience, The shop must have an excellent repair dept.

Big, high discount, high volume, chains are also best avoided as they generally don't give a crap beyond making a sale, any sale.

Good examples of truly excellent shops in London, UK, are Condor Cycles and Brixton Cycles. Both stock everything from high end road bikes to bare bones fixies to full DH/FR rigs. They both have excellent repair, custom bike fitting and general services.

http://www.condorcycles.com/store.html
http://www.brixtoncycles.co.uk/find_us.html

edit: Condor Cycles and Brixton Cycles both have excellent reputations with professional riders (pros and cycle couriers).