Having recently moved from Waterloo, Canada to London, I can say that some of it "just because" (market effects), and another part is the "benefits" you get here (compared to the US): free health care, cheaper university tuition, etc.

Have a look at banking/finance salaries in London, they are very comparable to SF/NYC.

EDIT: here are some highly scientific and factual polls on HN about London salaries from recently: https://news.ycombinator.com/item?id=5804134 https://news.ycombinator.com/item?id=10498333