Used it a few times when connecting to UA through LHR and there was never more than 25-30 people on them. And all of the flights I took were mid week flights so I would have expected them to be full particularly on the first departures southbound but they weren't.
That was a very small sample granted, but if there was a way to profitably fly the jets to MME then BD would have continued to serve the route longer. If they decided they can make money elsewhere then fair play to them that is their call.
Just out of interest, where did BD use the slots to instead?