This isn’t a rule enforced by BA and the crew members have made this up unfortunately. If the flight crew wanted to come out of the flight deck a phone call could be made to clear the galley of customers making sure the curtain is closed. The CCTV is there for this reason to make sure it is clear and the pilots are able to access this at any time. If you’re able to please feed the flight details back to BA to ensure the colleague is correctly versed in the procedures and not to potentially cause worry amongst customers explaining an event that happened previously.

There is a potential issue of how many oxygen masks are in the galley area in case of a decompression not being enough for all those in the galley so that’s something the crew has to consider.