Yes so I think for LGW it seems to be because they have switched the biometrics off atm. I assume the airside connections is still open so there is a need to check the person at the gate for the domestic flight is as per the bp and not a connecting passenger trying to avoid immigration.

At MAN T3 they had biometrics too which operated in the same way. However as they closed the connection points early in the pandemic they turned the biometrics off since anyone arriving must exit landside and everyone departing came from landside, so there is simply no need for the biometrics currently. That has continued to be the same after BA moved to R1. They are obviously continuing to not ask for ID at the gate as normal - unless there is a FLY prompt which needs clearing to check docs for an onward international flight.