Im in exactly same position as you, except Im 24 and have finished college. Also in London but would love to trade it in for life in SFO. Currently working for a startup here, but not sure about their future.
Im thinking the best route is to a) find a job with an american/multinational company and transfer b) stow away aboard a cruise ship
Im sure the gay scene in London is perfectly adequate for those who like that sort of thing...