Originally Posted by
USFlyerUS
To use either, you need to book AA flight #s in my experience. I don't think AA can sell tickets ex-LHR without the origination having been in the US (e.g., AA can sell JFK-LHR-FCO but not just LHR-FCO). AA can book a ticket with BA flight #s. However, you can't use AA trip and flight credits to do it, as far as I know, as AA is effectively acting as the TA for BA in that case.
AA is happy to sell you a LHR-SYD or LHR-TYO ticket -- as long as you are willing to connect in the U.S. Example: