Suspect it's a case of: pay your money and take your choice.
Clearly there is demand for flights from both BHD and BFS to LHR, LGW, LTN, STN and SEN.
Good!
Yes, of course those heading for north and east London may prefer LTN, STN or SEN. It's easier and quicker to get from Belfast to Aldergrove than from Heathrow or Gatwick to the opposite side of London.