IF Lufthansa exercised it options to buy BMI, I don't think that they would want the whole business and slots.
Personally, I would see their best option as being to keep sufficient LHR slots to operate a domestic feeder service to a LH/BMI operated LH network of say a dozen destinations. Lets say that this would require 50% of the existing BMI slots. They would then fund this whole venture by selling off the rest of the slots acquired to airlines desperate under open skies to get a toe in the door of LHR (ie. US carriers).
For LH to buy BMI and to operate LHR-Europe would be self defeating.
In this way, if it worked, great, if not, then it is a standalone operation easily severed. I'm not convinced that integration (a la AF/KLM) is necessarily on their plan at present.