I haven't flown into Heathrow for a while but...IMHO, BMI is in T1 while AA is in T3. If you aren't checking bags then I think that it is easiest just to walk across to T1, however if you are checking bags, then I suggest taking the Flight Connections bus, just to make life easier, however lines for flight connection can get very long, so I don't suggest it unless you absolutely have to.
Good luck navigating through Heathrow!
-AirBrit