I did query this with BA when I was operating at LGW up to 2004 and they confirmed there was no 'procedure' (no 'requirement' therefore?) for crew to notify the Captain that all pax were seated prior to push-back. The only 'check' was a common-sense understanding between the Senior and Captain. I had previously pushed back quite a few times with pax standing. My personal opinion is that there should be a mandatory check before push and I was surprised there was not. I'm pretty sure previous airline (DanAir) had both a head count and an 'all seated' call before push/start, but memories fade.
In BA our trigger for start and push was generally 'doors closed' and in those days the pressure on dispatchers for the mythical 'good boy bonus' sometimes meant the last few pax were shoved through the door which was then shut by the dispatcher.
I had one Italiano who actually boarded 6 'late' pax and then pushed my senior over onto the galley floor as she would not allow him to shut the door (at STD) until the pax were seated...................I got out of my seat and 'spoke' to him offering him some 'advice', and unfortunately delayed departure.
