I'm not clear on the question. Is it, "what is the best routing for a Christmas Day departure, when there is no nonstop available LHR-SFO?" Because obviously for a departure on any day other than X-mas, the best routing is the nonstop. If you intend to fly on Christmas Day, I agree with bmr12 that IAH is your best bet for the connection, for multiple reasons (best weather, best airport, shortest domestic connection).
$765 is an excellent fare. It might go lower, but odds are it will go up, not down.