How do you exactly know it makes money? Remember the numbers of bums on board does not mean it makes money!
Historically the early Shuttles were so busy they had a spare B737 to back the B757 up. So on a Monday morning instead of 2 B757s and a B737 back up they're down to a single A321 to catch the punters heading into town to start the working week. It was a high yield, high volume business route, not so much anymore. And yes, BMI WILL do well here I have no doubt.
Of course in those days GLA was a crew base with B757 drivers heading south to fly short haul from LHR so there was some flexibility in BRITISH Airways. If they can't be arsed to night stop a crew to hold onto a *loyal* passenger base then God help them. I don't want to fly on a low cost domestically, I always choose BA where possible.