Makes a lot of sense for BA to take ownership of bmi - from a business and logistical point of view.
My reasoning is as follows:
bmi operate an all Airbus fleet. The A319/A320/A321/A330 aircraft could easily fit into BAs Airbus fleet.
With LGW 'rumoured' to be going Airbus within the next year or so, then the bmi A319s could replace the B737 fleet topped up with some of the BA A319s from LHR.
The bmi A330s could replace BAs B777 aircraft at LGW. Cabin crew already do 'mixed fleet' duties, and the flightdeck could also do the same.
The bmi A320/A321s (as well as the BMED aircraft) would take the place of BA A319s sent to LGW.
The slots gained from the regions, could then be used for other routes out of LHR or put up for sale.
With regards to bmibaby and bmi regional, who knows. BA didnt want their own regional airline. bmibaby might look attractive to companies already operating B737 aircraft on a low-cost routes. bmiregional could attract interest from Eastern Airways, a management buyout or another European airline.
Either way, its bound to happen sooner rather than later with Open Skies looming. I think there will be more 'mergers' as such in the comming 12-18 months.