My experience in this area is confusion in the AA - BA comms. I’ve often booked with AA several months in advance. As AA roll the dice on their schedule, it can result in new BA PNRs, and further confusion. I’ve had duplicate bookings on same flight because of codeshares and other random oddness. But LHR desk have always been able to provide first leg boarding pass. Maybe that’s better agents at the First Wing, or luck.