Off another forum...
Lad went over with those German show plates on his car, the TUV/Police took exception to it and impounded it.
500 Euro fine
140 Euro release fee, only when he'd put on genuine UK BS marked plates... considering he didn't have any with him and you can't buy them in Germany, he was stranded and had to get some brought over.
Seems they've had some advice on what a UK car needs to be legal, and they're enforcing it.