Fall and Spring are both excellent times to come to San Francisco. The fog isn’t as bad as it is in the summer, and its not too cold (not that it even is compared to the UK).
You may want to try United ( http://www.ual.com ) or American Airlines ( http://www.aa.com ). Sign up for their “e-fares”, I’ve seen last minute flights to London from San Francisco for $300 USD in the past. Also, flights from London to Los Angeles will almost certainly be cheaper given that there are more flights and competition. It’s only a 6 hour drive, and if you take your time and take the coastal route, it can be very spectacular…..also Southwest Airlines has <$120 round trips from LA if you book early enough (and you could fly into Oakland, San Francisco, or San Jose)….( http://www.southwest.com )