Though the comparison of Lufthansa in Munich is that of a company that has also had similar debates and varying amounts of service from Munich, just as has Manchester, even does have quite a few services now.
The fact of BA's operations is that it relies of big business, and there simply are none of the international banks, none of the HQs of large companies that require international travel, and consequently none of the higher margin traffic that London has - that is a sad state of affairs about the UK rather perhaps than BA that simply mirrors such a state.
Additionally it says something about outer-London areas attracting such companies in terms of tax incentives, and the skill base that they offer.
bmi really don't have to support such a large organisation to the same extent - the corporate side of things that BA runs through executive club is far more extensive etc etc, and bmi will therefore continue to be successful at offering a lower-cost alternative to those up North who don't want to transit London.
BA and bmi both appear to have studied what people want and followed a strategy accordingly that matches the cost base that goes with it. Good luck to both.
Remember that with BA you pay for a structure that supports what the passenger pays for, and is wholly customer service driven (usually) - a low cost such as Ryanair often will not bother substituting an aircraft when it goes tech, but refund your ticket and leave you stranded.