A fair chunk of posts and threads in this forum essentially pivot back to this issue, directly or indirectly. It isn't new, BA had and has a large number of variants, even within the 777 fleet, so this isn't going away. It can change right up to the day of departure, though typically changes are less frequent in the final month or so. It's a constant juggling act, and whereas you are focusing on just 2 relatively short sectors with minimal requirements, BA centrally has to balance the rest of the network - in terms of flight crew, cabin crew availability, cargo, timings, fuel costs, stirage space - all at once. If this really annoys you the best answer is to book relatively late in the day, the surprises will be fewer the later you leave it.