As a data point, quite a few weeks back I went to Berlin.

It was a morning flight and very lightly loaded. I came down from GF to A18 I believe to see nobody there and it was at the ‘flight closing’ stage at T-26.

I did query with the agent whether the original or new time applied and he said they had brought forward the departure time, but would of course still adhere to T-20 to what’s shown on my boarding pass. The app showed it as a departure time ten minutes earlier too when I then checked, my BP in my digital wallet had not updated.

So, whilst I won’t risk it in the future, it does appear BA do bring forward departure times but stick to the original gate closure times if not everyone is there.

Not that this helps the OP however. T-19 is late whatever way you cut it.