There have been various accounts of why BA did not use the 747 on Los Angeles from the start, the most convincing of which is that it was beyond range of the initial -100 models. So how come Pan Am and TWA did so ? It seems that BA standards for range were somewhat stricter than theirs.
Several accounts from Heathrow ATC of how the summer afternoon departures of the two US carriers to LAX/SFO were met with some concern, given the unreliability of the early JT9Ds, especially if on easterlies. A TWA departure off 10R was described as "Leaving by the Piccadilly Line", and a Pan Am one was said to be a "Hedge Clipper". The moment the Rolls-powered 747-200B was available to BA, the first route it was put on was LAX, and SFO was started.
I don't know how often the US carriers had to stop for fuel westbound with their 747s. Presumably this was somewhat easier for them to arrange a relief crew to be at say Las Vegas to take over, where BA stopping there would be stranded.