The reason that this nonsense started goes way back. When T5 opened it was known that the baggage operation had to be in bins. The 757s that were still operating could not take bins so they went to T3 which could do both.
Others may know better but there is no rhyme nor reason why certain flights go from T3. The sooner that BA is out of that drab and dingy dump the better. Sorry if it incommodes the Lounge hoppers, but that situation is getting out of hand from what I can observe.
Still the whole system at Heathrow is so redolent of a task incomplete with bus boarding in this day and age. If they’re going to use T3 then do so and make sure that there is a jetway for every blessed flight.