[WillPS]

But why now, and why just these stations? I can understand why the decision has been taken, but I'd have thought they'd have also announced the closure of some of the Smooth transmitters, as I doubt they'll be doing much better.

If you mean 'why not sooner' this is a process which has been underway for some years now. Direction of travel has been clear, just energy costs will have hastened the end of it.

If you mean 'why not all of them', there are other factors other than regulatory commitments - transmission contracts in particular. Also don't discount the possibility that some of these stations are still viable.