As you point out, sleeping in a car on a public highway is not permitted (although lots of us have done it). Just park the car out the way somewhere rather than on a main road and it should be o.k.

My brother is currently on a short motorcycle tour of Cornwall, Devon and Dorset with a friend. Their method of avoiding paying campsite charges is to set up a tent on a beach, set up some fishing rods on stands and pretend that they are out all night fishing.