The A 380 had to be the choice for the larger aircraft as BA has to maximise the seats per slot in this bracket. If anything the current model is too small to give substantial seat mile cost benefits over the 747 and a stretch is both highly desirable and inevitable. The current model is the A 318 of the A380 family and carries a lot of the weight of a much larger aircraft.Its stretch potential up to 800 -900 seats is there.
The 787 is a clear 767-300 replacement and as result leaves a gap for a 350 seater twin which is where the contest between 777-300 and A 350 will lie.
In capacity terms the present order is pretty much a hull for a hull so allows little , if any, frequency or route coverage growth,- just 4% per annum based on aircraft size, so it's hardly exciting and indicates that BA will continue to be (over) dependent on its relatively slow growing Atlantic business and will not be going for the high growth markets in Asia from, which it has substantially withdrawn in the last ten years.
As for the need for additional capacity and parking around TBA , any expansion ever of the BA fleet in aircraft size and numbers raises serious questions about the sense of giving over so much of the former maintainence area to non engineering and even non aviation activities. Not safeguarding the space for future needs has always looked extremely short sighted on the part of both BA and the BAA, but it is unfortunately very much the UK way.