Clause 1.3.7 on page 178 of that document. The Slot Release Agreement contains a perpetual restriction on sale or transfer.
These slots are leasehold and not freehold, to use a property analogy. After three years you can make some changes to the leasehold property by redecorating without the landlord’s permission but you never own the freehold.
Also bear in mind that Virgin Atlantic itself had operated two of the three years with Little Red. If it was within a year of winning the jackpot of Heathrow slots as you suggest, it is highly unlikely to have thrown in the towel with that prize so close by.