At the very least, the government has to take on its debts (14bn in this case). Possibly having already guaranteed the debts will be paid, like they did iirc with train franchises.
As somebody else said above, privatise the profits, nationalise the losses. It's always the way.
The other thing is, it's amazing how people still don't understand that privatising effective monopolies can and never will work. It's not like you could ever choose your water company.
And Cameron was seriously thinking about privatising (selling off) the motorways... they just don't learn, do they.