The main advantage of the bay area is ofcourse the people, the collective knowledge, and the culture. You learn so much from other creative and smart people and bouncing ideas off of them (if you meet the right people, that is). On the other hand, it is very competitive out there and very tech centric so you can sometimes lose touch with the 'real world', so to say.
I have lived on both the East and the West coast and although I like the lifestyle in the east better, the west coast is the place to be if you are doing technology (the bleeding edge). You'll learn how the big guys do stuff and learn some valuable lessons that you can then use wherever you go (even if you don't end up living there longer term)
On another note, how is the startup scene in london ? My impression of it is that a lot of the startups are more business-centric (like on the US east coast) than tech-centric whereas it's quite the opposite in the valley.