Used to get club class from blighty to norway when the journey started in NI. Think it had something to do with journey length though, nothing to do with flying after or rank....
Of course, entitled really means only if the budget man can afford it. It is in QRs somewhere though, I think 3 hours might be the cut.
One final point, some Duty is jollies or expeds when you really cut down to it and so although the service pays, if it's exped funds they wont stretch that far.
You could of course travel superior class on HM AT.....