I think employment prospects are a little overrated on here. Certainly if you went to Imp or Warwick, either will get you to the interview stage if your application is good enough-- the London location makes little difference. The rest is down to you to perform. Some people have this strange idea that you'll walk in to a job if you went to a certain uni. That is not the case.
In terms of actuary, I can only talk about my course (Morse). It has provided the most number of graduates to this career over the last couple of years (of all degrees in UK). So again, the London edge is not significant. I know a lot of students here going into investment banking also (many having secured internships); its probably the most popular path. All recruiters visit campus anyway, as do they most unis.