This is no longer the case -- LAX-LHR on BA is consistently much better than SFO-LHR now. The new TBIT lounge is far better than the SFO BA lounge (which is very shoddily run), and the onboard service tends to be considerably better (BA feels the pressure from all the other premium airlines on the LAX-LHR route, such as NZ, while there is no competition from SFO). I used to use SFO all the time instead of LAX, but now it is no contest, LAX is better by every measure.