@Swindoneric (1604) and @BigDog. (1624)
If it is the same post LHR-OSL incident flight, as has been reported on Flyertalk, then the facts are slightly different from those which the poster over there has reported (he having been on board the flight in question).
Due to other cancellations/other events in London last weekend, nearest available hotels were in Brighton, so passengers were given a number of options when it was clear the flight was not going to get away in the small hours of Saturday morning (apparently after BA had served them dinner on the ground), and some of them did choose to leave the airport, head home and return for the rescheduled lunchtime departure.
Apparently there was overnight immigration (that’s how the passenger in question left the airport), so I fear BigDog’s third party coverage may have been wrong. The issue was – as you might expect for the hours in which LHR doesn’t have scheduled traffic, there wasn’t much of it.
One other issue on that question – the passengers on that flight included a number in transit who apparently didn’t have appropriate papers to land in the UK (again, source is the same individual on that flight); not sure how BA deals with that, but I would assume that latitude there lies with UKBA.