As I understand it half the time (if not more) BA's T3 flights are parked over on the old T1 180+ stands, so surely there's no difference between bussing people, or transporting luggage, between T3 and a remote stand and T5 and a remote stand? Apart from distance of course. T5A is a little further away than T3.

True that BA used to maintain its MIA flights at T3 so it could have a finger in every pie... but with so many of its competitors moving to the new T2 and T4, where BA no longer has a presence, it seems that factor has diminished in importance too.

Maybe it's just one of those things which we aren't meant to know the answer to?