NWSRG, I have found exactly the same situation. Heading to LA in March, and really wanted flights from Belfast. UA was working out reasonable, but was really wanting VS or BA to gain FF miles. From DUB the BA fare was over £160pp cheaper than from BHD.
With direct long-haul flights having reduced APD from Belfast, this distorts long-haul through tickets via another British airport. Would it not be fairer, as I believe the point of reduced long-haul APD was to save the EWR flight AND help with the leakage of NI folk to DUB travelling long-haul. So all long-haul through-ticketed flights be allowed to avail of the reduced APD.
Surely this is just giving UA a competitive advantage over their competitors that through-ticket via LHR, MAN etc. No really any benefit for the NI economy when NWSRG and myself, still see a substancial financial gain for flying from DUB rather than an NI airport.