The likelihood is that around 1,000 exchanges will be sold off in a few years time as size requirements diminish. For fibre services larger exchanges are already serving several exchange areas.
The 21CN VoIP project stalled, no big news information on why, but seems to be on hold for now.
By the way BT does not own the exchanges, they went under lease deal some years ago. So it would reduce costs, but they would not be a massive property windfall.
Another issue is the LLU market, which is reliant on the exchange buildings for providing unbundled voice.