12 aircraft is a bit of a small fleet to have multiple configurations within it, and would lead to some operating inefficiencies, which for the sort of capital investment each A380 represents is not something you want. There is a bit of an art in having this and then adapting your revenue management to maximise the take on each route served - an art that BA are doubtless well aware of.
Regarding what is now one of the largest remaining 747 fleets around, most of which are from the 1990s, there are a number of new long-haul designs coming along currently, and for such a big purchase that will be needed BA will doubtless have in mind to have the newer types and not one about to go out of production. There's a whole department, Fleet Planning, that manages this, and they crunch the numbers every day on optimising things. I'm sure that over at IAG the lesson of Iberia stocking up on a big multi-billion purchase of A340-600s which promptly lost much of their value and went out of production is well up in their minds.