The Simply Computing on Broadway ("downtown") is alright, but you have to talk to the right guy to get the right service. Their labour charges are quite high though.
The guys at MacStation downtown are quite cool, for the launch of Jaguar they actually had a real live Jaguar in the store. I wonder if they will do the same for Panther?
Every London Drugs with a computer dept. sells mac stuff. The bigger locations (like Broadway for example) have more stuff of course, but because they are more of a PC dealer, they lack a lot of the smaller stuff like laptop batteries, laptop ram and such. They do carry however a lot of Mac software and peripherals (with more being added to the list all the time). Oh yeah, I work there too.