The location of an assembly point is academic, as you both point out there is no management or control once they have walked out the door, nobody knows who is there so a roll call is pointless. Most will hang around for a few minutes to see if there is anything to watch or any chance of returning and if not they will go their own way. The key question is whether the external routes can take the flow of people out of the building whilst they play the first two verses of God Save the Queen without putting people in danger, eg from traffic rather than where they are intended to assemble. For licenced premises if all customers can safely leave the building and move a safe distance away to disperse then that is all that can be required in my view.