Young Paul, many of BMI's slots are not suitable for the destinations that Virgin wants to serve, so those slots would be surplus to requirements and would be disposed of in time - possibly in exchange for more useful slots from other operators.
Surely a look back to BA's takeovers of Dan Air and British Caledonian give you some idea of what would happen to BMI if it's taken over by another airline not particularly well-disposed towards BMIs (unprofitable) core market. Far better that BMI encourages further investment (and board supervision) by Lufthansa and SAS, or solicits a buyout from an alternative organisation whose motives would be established before any sale as being to improve and expand the current BMI operation - including longhaul (though maybe not to the US for now!).
Looking for help from Virgin is almost as daft as looking for help from BA! But, as I work for Virgin, I'll happily accept those slots if you really want to give them to us.....