I don't see why IAG would want to buy BMI mainline. It needs slots at LHR for BA to expand. BA has gone through 10 years of cost cutting solving a pensions issue twice, introducing new rostering and conditions and in general simplifying it's business.
Why would it add a BMI as a loss maker and increase complexity that has been solved in BA? It would be far simpler to buy slots or let BMI contact on it's own freeing slots and making skilled crews available to recruit.
Lets face it other than BA, who else would want that volume of slots at LHR? Virgin hasn't got the money. I would expect a few foriegn airlines interested in a few slots but not the number that BMI has. The UK isn't large enough for 3 mainline carriers at LHR, plus all of the low cost short haul across UK. BMI are trying to compete with BA on niche medium haul and both BA and Easyjet on London - Europe. Unfortunately for some great people at BMI, the market has probably moved on.