Alan
I think you may miss the point. If NATS takes the fee for that flight between Plymouth and Edinburgh, NATS should provide the service. But it doesn't. It can't.
By contrast, if you want to fly IFR from anywhere in Germany to anywhere else in Germany, you fly pretty much in a straight line and a DFS controller provides you with a radar control service. For this, they charge a fee. Fair enough, I think.