Unless one has a real reason, (needs the miles, fare much cheaper, whatever) no one would choose to transfer through LHR with BA if one had a nonstop alternative. Even with the advent of T5, flight connections/transfers are not easy nor pretty, just go take a look at the BA forum where the best advice given is to actually exit customs AND to check back in as that is seen as a better move (and you'll also see people complaining about slow security at T5 in any case). Again, those that ned to go to a variety of Euro destinations generally choose airlines other than AA.