Hello experts!

On all my recent BA flights, on the return flight back to London my boarding pass has beeped red at the gate with “unable to board” and the agents will take my passport and press a few buttons on the computer before I can board. I think there was a message in the app saying my passport would need to be checked at the airport due to rules of my destination country (the UK). But no one else seems to have this problem and I also don’t have this problem departing London. Anyone know what’s going on here?