Regarding point to point London traffic, STN could well be a convenient alternative for some, from Belfast. However from the point of view of connecting traffic, NI only has BD and EI to LHR. EI only has codeshare with BA, (and the daily EWR for the Americas).
NI has no alternative links via Brussels, Amsterdam, Frankfurt etc like many other UK regionals. LHR is a necessity to keep NI competitive business wise, and for long haul leisure traffic feeding into Star. Surely with EI the only competition with 3 daily winter into LHR, and NI seperated from the mainland, this route should be a cash cow for BD??