bmi are going to find this to be a very tough operation. They will have a hopeless frequency for the business travellers and they will operate an aircraft that is far too small for this route. It is entirely likely that this route alone will add many millions to the bmi annual loss. It smells of a train set operation rather than a serious business activity. I guess that if BA had been given these frequencies they would be able to generate around three times the revenue per flight that bmi will generate. That calculation is easy. Thery have first class and large business class cabins and they have a very strong freight operation. bmi have no first, are likely to have a small business class and their freight marketing is pathetic by industry standards.
As one who is well experienced in the ways of the CAA I would incline to be cynical as to why bmi have been given this route. It is a certain thing that if bmi fail on this route they will face massive hurdles in getting more long-haul outes out of LHR.