I'm not sure the issue here concerns the technicalities of BA's contractual responsibility to passengers when there are deficiencies in the ground handling operations at Heathrow; it's what possible claim OP would have for £££ for breach of contract in this case.

OK so disembarkation took a while. At what point is 'a while' a breach of contract in this context? OK so OP expected what amounts to priority disembarkation but got the opposite, but isn't that just one of those imaginary benefits - something that might be expected or anticipated but not something baked into the contract so that non-delivery gives rise to a legal claim.

i can't see OP being due any cash, any more than I would expect BA to compensate someone whose short-haul flight parked at a C gate, thereby delaying exit from Heathrow by a few minutes. An apology might have been nice, but given it wasn't forthcoming I wouldn't advise OP to waste any more thought or effort on this one. S/he has had a therapeutic rant here, if that is any help.