The only serious attempt to make BHD LCY work was in 2007/8 when Cityjet/ suckling did 328 on the route. Nonstop flights, quite long sector on a prop. The broader context needs to be considered for that time;
Aer Lingus entered Belfast London market with new BFS LHR route x 4 320's per day. Ryanair entered BHD STN with 5 daily 738's on the route. London was flooded with seats but more importantly few knew that the LCY route was operated and fares were ridiculously high.
It has potential as mon to fri plus Sunday route - but we shall have to wait and see. Business orientated , but I'm not convinced. They will invest a lot of time and money making this route work.
I agree with previous comments that BE has moved from one high cost airport to another , except this way they get a £25m payment plus escape the rigours of intense competition on every route, ie EZY on IOM, EI on BHD etc.