Whilst the 77Ws were part of the package for the delays to the 787, I believe that BA found them to be far more efficient and profitable than they had expected, hence the top up orders over the years which have slowly grown the fleet.
I remain a little surprised that BA doesn't seem to have done anything yet with the 777X. I know that they have a number of A35K and 787-10 on order but between them they aren't going to be able to replace all the 744s due to be retired and the early 777-200ERs (which are as old, or older than the 744s).