Virgin, British Airways, and United all fly direct SFO - LHR. I'd take them in that order, too. BA's inflight service is friendly and efficient, but Virgin is all of that plus fun and hip. Plus, they've got better inflight entertainment.
If you don't mind connecting, check Continental through Houston or Newark. They are consistently ranked the best US airline for long-haul international routes. They fly to both Heathrow and Gatwick, and you shouldn't be afraid to use the latter. Their baggage handling is MUCH more efficient, gates are less congested, and the Gatwick Express rail service whisks you straight to central London with no stops.
Have fun! Where do you think you're going to cruise?