Misunderstanding accountancy there...
Dull but important: Freehold property = asset. Rent = liability. If you sell an asset, your size (as measured in money) reduces, and is reduced further because you now have a liability to fund.
This could have two motivations: 1) "Look what we've done Mr Cameron, we're much smaller now", or 2) "Look at the cost of rent Mr Cameron, please can we put the licence fee up?"
There are also considerations of maintenance costs if the freehold buildings are old and in need of a lot of work, and if the Beeb are now shedding thousands of jobs it's better to sell an inflexible building and rent flexible space instead which can grow or shrink as required.
So not a bad thing then really.