The reason you're getting very bizarre answers is you've probably unintentionally asked a "what happens when an irresistible force meets and immoveable object" type question.

Generally BA and AA are restricted due to antitrust concerns from collaborating on TATL flights, which is why you can't earn or redeem miles on such flights. But that's USA to/from LHR as a general rule.

So the question then is, which flights are you talking about, specifically? Which city pairs? The only US-UK codeshares between BA and AA that come to my mind are the MAN ones, and I'm having trouble thinking of an AA metal 5th freedom flight from Latin America to the UK, for example -- but if you add detail on which flight you mean you might get a clear answer more easily.