Belfast City was another route I used from LCY in the past. Unfortunately the loads were just too thin, despite FlyBe having good marketing presence at the other end of the route. In the final year the Dash 8 was sometimes down to single figures for passengers.
I guess Belfast does not have a substantial London-dependent business market, and seems to be particularly price sensitive. The low-costs, particularly Easyjet, have come to dominate the market. BA, after all, dropped London to Belfast altogether and although it may be said they just wanted the Heathrow slots, note it was Belfast they pulled rather than anywhere else.
It's also a long haul for a turboprop. It's just perception though, because if you left Belfast City at the same time as BMI you could be on the ground at LCY while they were still in the Bovingdon hold, but it just seemed a long trip. Edinburgh in Scot's Dorniers seems much faster - they must have a higher cruising speed than the Dash 8.
The detailed market mix of a destination is very significant to LCY - for example, Edinburgh does so well and Glasgow so badly. There's business traffic in Glasgow, but seemingly not the right sort of business.