Is the 'static' part of the regulation fairly recent? It's a long time since I studied the T&C Planning system at university but I don't remember that part of it but do remember my lecturer illustrating it by referring to the adverts on vehicles you see littered about fields alongside major roads. Perhaps the LPA just aren't interested in pursuing those sorts of breach.

I believe it is also 'illegal' to place 'for sale' signs in/on a vehicle on a public highway.