Terminal 5
It is all well and done saying that BA should concentrate all of their flights at Terminal 5 but T5 WAS orgionally intended to take all of BA flights at it's planning stage but by the time it was eventually built it was already obvious T5 would be full when it was completed so several BA flights had to be in T3.
Now saying that BA currently have a split terminal operation which T5 was supposed to eliminate what is going to happen when BA absorb BMI those flights will not really be able to be accomodated in T3 untill the remaining Star Alliances airlines move to the new T2 in 2014.
So on completion of the BMI takeover BA will have flights from T1, T3 and T5 and BA will be right back to where they were before T5 was opened having flights from three separate terminals.