One word; insurance. As with an import, generally speaking it'll cost more to insure a cat. C or cat. D car over a straight, UK car. The "worth" of such cars are therefore offset to make that car more appealing to buyers.
If I had the option to buy one of two identical cars, one a straight, UK and one an import/cat. c/cat. d, all else being equal. The UK car is going to cost £300 to insure and the other £400, I would always choose the UK car. If the latter car is cheaper though, it then becomes more appealing.