All of the above is also true regarding Silicon Alley in New York. Just visit expatistan.com and do a comparison. Housing is actually slightly more expensive in New York. As for your point two, Wall Street pay is not what it used to be while software engineering salaries outside of banks have been increasing steadily for a couple of years. Nobody doubts NYC status as a startup hub anymore.

Point one is also increasingly true for the SFBay area itself, but there are not enough banks there for them to be a problem for startup recruiting. You could also do a comparison on expatistan.com and see that housing is only 7% less expensive in SF than in London. However, IT salaries there are probably the highest in the World.

If current trends in London continue: startups start to pay more and banks keep laying off people. This is probably a good thing.