The registration authorities probably have some stupid regulations about not making body panels out of plywood and bog...
Bit like the awesome Batmobile from the first Batman movie, built on a stretched Chev chassis...apparently the whole body had to be lifted off to get to the engine to work on it, and because it was only slapped together for the filming, once you got over 40mph or so, it got really scarey to drive.
Looked awesome though...and totally unlike the "Tumbler" in the new Batman movies, which is the real deal and capable of over 100mph over all sorts of terrain. That's what happens when you let a British film company at something...they don't tart it up to simple outward appearances like Hollywood...they usually make the weird vehicles actually
work in real life...