The gap between London and UK-but-not-London salaries is probably pretty comparable to the one between London and San Francisco, but most people won't move a few hundred kilometers to London to get the extra money. The proportion of people willing to move across the ocean, leave behind friends and family, and embrace a new culture, is lower still.

But I think the other commenters have it right: when British people dream of living overseas, they dream of Australia, Canada, Spain, and the south of France.