And if you happen to come accross a "Zulassungsstelle", which is where you register all kinds of vehicles, there's plenty of shops that will print pretty much anything you need. Every city has one, just ask for "KFZ Zulassungsstelle". A German license place is worthless without the official seal on it, the one between the city part (KA for Karlsruhe) and the individual part of a license plate. If I remember correctly British license plates don't have that so you shouldn't get in trouble with the police. The downside is that as spooky has said Germany closes down over the weekend you won't be able to find anything before monday morning.
"Life is not a journey to the grave with the intention of arriving safely in one pretty and well preserved piece, but to skid across the line broadside, thoroughly used up, worn out, leaking oil, shouting GERONIMO!"