Slightly OT but we embarked on our very first cruise last Friday. BA could learn a thing or two from Cunard. People attempting to queue jump to earlier boarding than their class of travel or status supported were very politely but firmly told to sit back down. It’s admittedly a very different thing boarding 2600 passengers on a ship over the course of 3 or 4 hours to boarding 150 people in under half an hour for a short haul flight but if you don’t have the will or the training to stick to your advertised processes then it’s just not going to happen. The staff, as far as I could see, were contracted from the port of Southampton not Cunard employees so the third party excuse doesn’t hold.
Just send people back if they attempt to board too soon as emeraldflyer witnessed. Or have the turnstile just not allow progress if your group isn’t called. But BA have to care and I think they mostly don’t.