Then there's the people from BRS,BHX,EDI,GLA,LBA etc now going through DUB. Over 15% of pax crossing the Atlantic from Dublin every day originated in the U.K. and that number is growing.
I suspect that APD has something to do with that...I have actually flown from DUB to LHR before going transatlantic. Booking on a through ticket from DUB rather than BFS was £1000 cheaper for a family of three to Toronto. All because of APD. And the same works the other way...fly from the UK to DUB on a short-haul flight paying low level APD, then international from DUB with zero APD.