I'm not sure there are actually any 'green belt' laws, as such, just guidelines:

"The Government sets out its policies and principles towards the green belts defined by local authorities in England and Wales in Planning Policy Guidance Note 2: Green Belts. Local Councils are strongly urged to follow PPG2's detailed advice when considering whether to permit additional development in the green belt, or to assent to new uses being made of existing premises. In the green belt there is a general presumption against inappropriate development, unless very special circumstances can be demonstrated to show that the benefits of the development will outweigh the harm caused to the green belt. PPG2 also sets out a number of examples of what would constitute appropriate or inappropriate development in the green belt."

Councils appear to make it up as they go along, and 'green belt' is a term used most often by estate agents.

The Council in question is quite capable of letting the buildings remain, but in all likelihood are under pressure (real, implied or anticipated) from elsewhere.