"Probably because the AC lounge is in Terminal 3 (where the AA flights arrive/depart), and the best of the BA lounges are in T1 and T4. Or more to the point, as we have seen from the "experts" here, not many people are aware of this reciprocal access availability."

The BA T3 lounges are quite nice and v quiet ime since there are only 2 BA flights a day out of T3. I had to travel recently with AA (urgh) and used the BA 1st lounge since it was far nicer n less busy than the AA Flagship Lounge

Dave