@Neptune: even test-cars and prototypes have to have plates (I think, in the UK it's the same as in all other European countries), when they are driving on public ground. It doesn't matter, if the have a regular registration or just dealer's/garage's plates, but there must be any kind of plate.
Only on fenced, private ground you are allowed to drive vehicles without registration.

@G-Mann: I guess, it's the most simple reason: te guys are doing something illegal.

How are the punishments in Britain for these delicts, riding without, with invalid or faked plates?
In Germany they are quite hard, because over here a license plate has the same status as a document (due the silver sticker on it), so it's similar, as you were faking a passport.