It is a mess and I agree there is no self administered jurisdiction of the airspace but they do have the right to legislate on it if desired. It's just cheaper to devolve management of their airspace to the CAA. Overflights by Commercial Air Traffic or aircraft carrying out Aerial Work were always going to be exempt. Remember we're talking about recreational flying only.
Like England up to the present restrictions, GA is not illegal, it was stopped simply by utilising the travel ban that still currently exists in Wales. Whilst you can drive any distance in England, it's still supposed to be for an approved purpose. In Wales currently 'staying local' (whatever that means) is still being enforced so as I said, if you wound up having to explain why you are sitting in a field you'd have few friends to turn to for support.
Perhaps this says it better than me:
Wales: Later