"has NATS itself taken the decision to close UK CAS (with advice from its own lawyers), or has it been leaned-on by HM Government?"
Decisions like this were take years ago. When the event occurs the plan is implemented. You do not wait until an a volcano erupts an the plume start blowing towards you to debate the relative merits of open/closing the CAS.
Planning for a nuclear war/tsunami/asteroid strike/pandemic flu or whatever you have a contingency plan and you implement it. The words of the press release, the speech of the PM or whatever - it was all written long ago.