It'd be hard for me to go back, for a number of reasons. Admittedly, a fairly key one is that I detest the political climate of the US and that ignorance seems to be something ennobling there. Also, I really am quite in love with London (and England, although I doubt I'd stay, if it weren't for London). There's just so much to do here and it's such an amazing mix of culture, cultures, history, and a ceaseless hustle-bustle of human activity. London puts pretty much every other city in the world to shame, in my opinion (although it'd be nice of things stayed open later). Other people have highlighted some other key reasons (access to fabulous vacations, a healthier culture, etc.)
That said, this is a rather abysmal place to be in the tech industry. Although I'm fortunately doing well, the industry here is significantly smaller and the pay and career potential are significantly worse than back in the Bay Area. In general, I think opportunities here are pretty crappy, at least in the industries I see. If I were faced with a period of unemployment, I'd probably have to move back, if I wanted to maintain a remotely comparable standard of living.