Belfast-City Airport (BHD) is only served from Heathrow and Gatwick now in London.
With BHD being substantially more convenient for pratically anybody in Belfast, I think a lot of people, if not the majority would still prefer going from BHD to LHR or LGW rather than travel out to BFS to get into STN, LTN or SEN if they were heading more to the north or east of London.
I think BE would do well on a STN-BHD route. BmiBaby were the most recent to operate it and were reportedly very pleased with its performance.