Because, silky private healthcare providers have been providing NHS services since the last LABOUR government opened it up to them. (I was very, very involved with the process for a trust at the time) It doesn't mean that private healthcare providers have bought or own any part of the NHS. It means they hold a contract to operate services on behalf of the NHS and under it's supervision for a limited time.
Old buildings have always been sold off to fund new ones in the NHS. For example, in Sheffield the old Jessops Maternity Hospital was sold off and the new Jessop Wing opened in 2000. Some parts of the old Jessop wing were knocked down and new buildings put up by the buyer the University of Sheffield. The Victorian part is now the Uni's department of music as they can make use of lots of small poky rooms for music practice that are effectively useless in the modern NHS. It's a lovely development, you should google it. In return patients got a brand spanking new maternity hospital fully equipped for modern medicine and in tip top maintenance rather than have patients in leaky, mouldy, draughty rooms and operating theatres frequently shut down because of maintenance problems or wards closed off and beds out of use for the same. Didn't see the same hysteria when Labour did that.
So in answer to your question no, I don't think it is being sold off. I think more involvement of private contractors is being brought in to bring it more into line with European systems which gave much better health outcomes.