Quotes:
"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."
"
So on completion of the [Dan Air/B-Cal/Brymon/Manx/etc.] takeover BA will have flights from T1, T2 and T3 and BA will be right back to where they were before T4 was opened having flights from three separate terminals. 
I sit to be corrected."
That may sort itself out in the long run as LHR-1 is due to be demolished and rebuilt once the new LHR-2 opens. If (as is likely) BD has been completely subsumed by then, it will not be an issue and BA will still be split between LHR-3 and LHR-5. So it's little different from the days when they were in LHR-1 and LHR-4.
It was a bit silly not to have LHR-5 big enough for all of BA (suspect it's a lack of stands rather than the main building itself), but they have manged to squeeze IB in there.