Well, at the end of the day, somebody will be operating those slots. And bmi crews operate more of them for less money more effectively than most other operators at LHR. So would you kill a goose to get the golden eggs?
Why would Virgin sell slots when it might want them in the future for a HUGE longhaul operation, and they are (in broad terms) covering their costs at the moment? Makes more business sense to use them as and when - change would have to be gradual - you can't start 20 longhaul routes overnight. And even if you could, to rule out the possibility of starting 20 more next year would be crazy .....