I can remeber the whole reason for setting up the EA (or NRA as it was then) - to catch out lying water authorities! If it was a good idea then (and it was) why isn't it now? One word. Cost. The EA would prefer to do things that mean they don't have to leave the office or have too big a fleet of vans.
Mind you, I think it would be a good idea to introduce this to traffic policing. Just hand over the control to us, the motorists, and when we run a speed camera, we'll just stop and put £30 in an honour box by the side of the road. This will eliminate the need for pesky paperwork. Think of the saving!