For BA departure is when the aircraft releases its brakes on stand in preparation for pushback.

There are three different measures of departure punctuality at BA.

Ready to go (the current focus measure) - this is when all the doors are closed and to trigger RtG all doors have to be shut 5 mins before departure.

On time - Pushback occurs on/before the STD

<15 mins - Brakes released / pushback up to 15 mins after STD.

It’s this last measure that the industry often uses to measure punctuality.