I believe that in the past, when this proposed BA/AA tie up has been on the tables, part this issue has been slots at LHR. Somewhere recently someone from BA has said that in the current market, with the Open Skies agreement, they have accepted that losing slots in order to get this alliance is worth it.
BA also have their party trick hiding behind the bushes remember - OpenSkies. They are now able to compete with Air France-KLM and Lufthansa in their own territories, without relying on LHR. Losing some slots here and there isn't going to make much of a difference if at the end of the day, they're going to be better off.