Sounds like the member of staff you spoke to is not happy about this "Supervisor" and would like you to coomplain to BA to highlight what has happened, as (i'm guessing) it may be tricky for her to do that. Some outstations do not operate the way they should, or as BA wants them to and sometimes seem to make up their own rules...It should not be like this, but I think you should write in if you don't mind.
An example from our perspective... An outstation manager or supervisor decided 4 mins was long enough for us to security check a 747, and proceeded to start sending pax down to board. That was an interesting conversation to witness. Oh and this was to start boarding an hour before departure.