Thanks for your help. This is actually part of a LHR LAX PPT trip sold on Expedia on 13 December. The earlier BA flight is 20 pounds cheaper but leaves at 10.25am (10 hour layover in LAX) whereas the AA flight leaves at 12pm (8 hours layover In LAX). I think I'll get the AA flight as it leaves later although my chances of being op-upgraded on this flight will be lower than on BA...