Do you have the AA reference as well as the ba one? Sonetimes going direct to american airlines website will let you choose seats as american offers free seat selection once youve booked so you dont have to wait till 24 hours before. My experience has unfortunstely been that these types if bookings often dont work for online check in. Sometimes if you call up seats can be assigned within 24 hours if its a ba flight from lhr if check in is unavailable. You would still need to check in at the airport however.