I've spent a good deal of time in London and live right down the road from San Francisco. Don't get me wrong, San Francisco is a great city and I love spending time in it but you clearly don't know what you're talking about. London is a major economic and cultural hub, the capitol of what was a globe spanning empire dwarfing anything that has followed it in influence and significance, a current major fincial hub, the current capital of one of the most influencal and wealthy countries in Europe, home to immigrants from throughtout the commonwealth and beyond, has been the center of major artistic and musical movements and etc, etc.
San Francisco is a very dirty little city by the bay whose culture high point was hippies (note: I still love the city).
The big American comparison to London would be New York which i'd take over London any day (Of course i'm a pretty patriotic American so....). In terms of cultural significance San Francisco doesn't stand a chance versus London.