BA and Lufthansa could just agree a deal for BA and bmi to swap slots. BA would get all of the Heathrow slots, bmi would simply move to Gatwick with a load of cash presumably equivalent to the price which Lufthansa is paying for bmi to start with. There would be no competition issues as bmi would still be flying the same routes from London - they look at the market, not the airport. Much more likely than an outright sale, which would be mired in Takeover Panel issues from here to kingdom come.