While not disputing the fact that BMI's A330's could generate a greater revenue from LHR and there fore a potentially more profit, there are also other issues to take into consideration. There is far more competition at LHR than MAN, the more operators on a given route is surely going to reduce the amount of money it can make.
If it chooses to launch a route to a new destination with a airframe taken from MAN then it needs to be sure that it can make money on the route in a shorter period of time as possible. Added to which if it does not replace the Manchester based airframe then it has to fore go the revenue it generates and presumably that routes contributions to it's overall profits.
While not just a BMI issue higher yield front end services surely need to be offered from Manchester, if they are not then how are you going to grow the up take of both first and business class.
I seem to remember BMI announced a intention to ordered some five A330's in July 2007 what has happened to this order are they delayed or cancelled.