It's unlikely that the flight would have continued to LAX if the problem had occurred at the same time in the reverse direction. The Captain would have first call on the decision but in consultation with the company I'm pretty sure they would have returned to London.
The aircraft would have to be returned from LAX empty on a 3-engine ferry and the passengers booked on that flight would have to be rebooked elsewhere. Far easier to turn round, send the bust aircraft back to its engineering base, put the pax on another BA flight and save a stack of money in the process.
Also, the flight over the Rockies to LAX will involve flight over high ground and assuming (as we always do the) the worst case of a second engine failing, you might be poorly placed. So, best bet all round is to go home and try again.