I've tried to put together various 2 + 2 together here.
Part of the deal for BA taking over BMI is that a number of slots will be given to another carrier to provide a competitive service on routes to Edinburgh and Aberdeen. However, among the conditions is that the slots will not be given to a carrier who already has slots of their own leased out and therefore they are under-using what they already have.
Virgin has had slots leased out to other carriers and would therefore be ineligible to apply. However the lease came up for expiry and Virgin are taking them back. That means they are now OK to apply for the Scottish slots. What they need is a slot-sitter for a while. Manchester is about the shortest route that looks remotely sensible so go for that - for now.
Having done this, they are now eligible to apply for the Scottish route slot transfer. Apply for and get those - doesn't really seem to be any other meaningful interest. These slots are only protected to the Edinburgh/Aberdeen routes for three years, after that you can do what you like with them.
So, Virgin will have somewhat infrequent (compared to BA) services to Manchester, Edinburgh and Aberdeen, ticking over and losing, well, not a lot we hope. In three years time they can dump all these domestic routes and do what they like with them. They can start their own services, doing what their business does properly, namely long-haul, or they can lease them out again once the time lock expires.
Look out for the press release around the end of 2015 saying they are "reluctantly" withdrawing from domestic routes due to [choose one of] high APD / their own environmental concerns about domestic flights / desire to mount direct services from Manchester etc with the 787. Goodness, the draft of this press release is probably already written and sat in Ricky's safe for a few years.