Do aircraft ever leave after engine runs? All the time. If there was no chance of that happening do you really think BA would waste the time of the customers and staff for a bit of a laugh? Of course there are scenarios (such as yours) when in fact the problem cannot be quickly rectified. I take your point about the rebooking process. If there are seats for everyone on the next flight we can transfer all of the passengers at the touch of a button. That doesn't work if there is not room for everyone though, hence the first come first served line. It is never going to be pretty though, no matter what happens BA will never be able to anticipate everyones choices and needs.