Hi, I've worked in a couple of startups & agencies around the British Isles and one investment bank in London. My experience would convince me to advise that you should work were you want to work, but in general my experience is:

1) if money is the prime motivator, work for investment banks, preferably as a contractor; 2) if you want a breadth of experience, work in a good web design agency; 3) if deep domain knowledge is what you want, work in a startup.

(Of course, you can't do the contracting without a few years experience in any of the above.)

The points are all a generalisation but this final one is not, it's not what you are doing, how much you are getting paid or where you are doing it, it's who you are doing it with. The human factor is, in the long run, the most important thing. Are they smart? Good, you'll get smart too. Are the decent human beings? Good, you'll live a long life.

Humans: the cause of, and solution to, all life's problems :)