You've got to remember that LHR, just like any other airport, is a shopping center with a few runways attached. The BAA do not want little aeroplanes in LHR. Look at an A380 next to an EMB 145 on the ground and you will see what I mean. They take up roughly the same 'space and time' on the airfield (within reason) but they have massive differences in terms of revenue for the airport operator. They have been trying for years to price small aeroplanes out of the airport. They even charge them all the same landing fees. You can't get enough people on a 'small' aeroplane and get enough from each seat after deduction of charges to make a profit. No one can and Virgin will not be able to either.
Getting BA to hand back slots to protect services to Scotland was a big mistake. A better outcome would have been to get BA to protect all the slots and maintain the same number of seats. As has been pointed out somewhere else if the slots don't get used they will probably go back to BA who will then have no obligation to operate the service. Either way BA are going to win.