LHR-SFO is operated vy a Virgin B747 and BA 747, however if going with AA or UA then you will get your 777.
JFK-LHR with Virgin is a toss up between any of their types, and it will be pot luck whether you get a 744, a 343 or a 346.
SFO-IAD may well be a 777 with UA or AA as well, Greekdude is probably the best person for that one though.