I could be wrong but didn't the BBC only sell the lease hold? If true then the BBC owns the free hold.
The BBC didn't sell the land that TVC
sits on so they own the freehold. The property market when the sale was first being mooted was depressed so they decided it wasn't a good idea to.
The TV Centre buildings were sold to Stanhope, who presumably pay a small amount of rent to the BBC
BBC Studioworks lease back part of the building from Stanhope.
They then rent out some of that space to ITV, as they rent out studio 1 to others
It's like how leasehold housing works - the land is owned by a freeholder and you just buy the leasehold, normally for 100+ years. You pay the landlord a peppercorn rent. You can modify it, with permission of the freeholder and you are then rent out a room to a lodger