Definitely get a BA number if you plan to fly BA LON-US. As you should know, BA flights between Europe and the US earn no miles/points on AA. The one exception is MAN-JFK, but you must be ticketed on the AA codeshare, with the AA flight number. You can fly to canada or Mexico and you're alright, but if you were planning to go direct LON-US, you'll need to either fly AA metal or get a BA number (or use another OW partner, such as IB).