Shed
Whilst I agree with you about the games that airlines play at LHR and other slot-constrained airports and that VS are using the slots for MAN from their own allocations, it would appear that they are hoping to get some of the "remedy" slots that BA are having to give up.
This comes from their press release
Sir Richard Branson’s airline is going to apply for all of the remedy slots being awarded by the European Commission following the IAG takeover of bmi, but is using some of its existing slots to service the Manchester to London route. The airline believes competition on this route has been neglected in the remedy process and aims to provide choice for the 650,000 passengers who travel between the two cities.
Manchester | Virgin Atlantic
VS are bidding for 12 pairs of slots - see here
Virgin Atlantic takes on BA on UK routes - Telegraph
and as I understand it hope to operate some LHR-Scotland flights, also starting in Spring 2013.
Suzeman