All WW wants out of the deal is the slots for expansion of BA LH routes from LHR (the slots themselves being traded with other airlines to optimize schedules). BMI mainline has its own legacy airline costs, and I'm not convinced its operating costs will be sufficiently lower than BA mainline to warrant running two parallel operations from one hub with a "BA Express" type operation.
I think the BMI aircraft and slots will gradually pass over to BA mainline to fuel its expansion, rather than the other way. Crews? who knows. Probably an invitation for selection to mainline or voluntary redundancy.