BA obviously have some clout, as the original plan was for everyone to be biometrically scanned on the jetbridge as they exited the plane and then exit the terminal through the biometric scanners at the north-west end of passport control. BA have made MAG put in a specific non-biometric arrivals route, even if it is bussing it is probably still more efficient than the biometric route.

It's worth noting EasyJet domestic arrivals in T1 were always bussed in exactly the same way that BA domestic arrivals are now, even if the plane was at a contact stand.

There probably isn't much else that can be done now. The layout of the airport is baked in. The design was based on using biometrics which BA rejected.

Others have said that Aer Lingus are using the biometric route, scanning everyone as they leave the plane. Does anyone know how that is working with a full A320? If they're struggling with 50 people on an ATR72, I dread to think what it's like with 180 people.