That's also true, though I doubt it makes that much of a difference to salaries. Simply put: I could quit my job today and have a month of interviews lined up

That's also true, though I doubt it makes that much of a difference to salaries. Simply put: I could quit my job today and have a month of interviews lined up in a few days in SF. In London? Not enough companies to sustain that. That leads to companies competing for talent on wages. London is great but it never really got a tech boom like SF did.