As you say, it does appear to be aimed at trailers/caravans. We often see both, trailers often have different number plates from the tow cars and caravans have a piece of cardboard stuck on the back with felt tip lettering applied.
However, I think they should aim at cars with illegal number plates too. On a recent trip back from Cornwall we lost count of the number of cars that passed us with illegally spaced numbers and letters. At one point, 3 came past us one after the other.
Randa