London is a rather expensive place to be bootstrapping a startup. What would make it more appealing than Birmingham (nearby, pretty big), Edinburgh (medium-size city, with one of the best UK universities) or any other of the big cities with universities?

If you care for frequent flights to US/Asia, but don't mind the language barrier, Amsterdam or Berlin would be good choices as well. (Living cost in Berlin is relatively cheap yet it's attractive enough to live in for people - Amsterdam is expensive but small enough to commute if you want to live someplace cheaper).