Coming to London to buy an overpriced road bike : Where to shop?

It's fairly common for people to trick out the big ticket parts of the groupset (cranks being the most notable), and then to cut corners on the consumable parts (cassette, chain) and the parts that others may not notice (rear der).

I agree about Cassette and Chain, but I think everyone notices the rear derailleur. To me the most 'invisible' components after the aforementioned are the front mech and brake calipers.