What is a reasonable salary for living in London? How do you Londoners feel about your city?

I have a job offer in London, I qualify for a tier 1 visa. I do okay here in San Francisco, which is notoriously expensive. But London seemed even more so. What is a cool and inexpensive place to live(any place comparable to the mission or SOMA for anyone who's spent time in SF), where are the technologists? London seemed to be a very diverse and nice place. What is the general sentiment about expats from the states? I've done a little research and read a lot of threads on here but I figured I'd ask real Londoners anyway. Thanks :)

Edit: Thanks for all the replies, this has been extremely helpful!