I always used to have German plates (with my UK reg) on my VWs but it got a bit much with the police often giving me a friendly ticking off (or very rarely a ticket if he/she was in a bad mood), but I can't even find anywhere that does the German plates now, even the plate pressing guy at Bug Jam this year only did the standard UK plates, and would only do exactly what it said on the V5 and nothing else as he was putting his postcode on the plates and didn't want to get prosecuted. I can understand that!
Then again, I see plenty of 4x4's and suchlike round my area with carefully crafted spacing and artistic use of yellow/black plastic screw tops to make letters etc., but they never seem to get pulled, and annoyingly I always see the same ones driving around scott free.
Personally, I won't risk it anymore (maybe I'm just getting old and sensible) as I am pretty sure failure to display a valid VIN is £100 fine and/or 3 points (from watching Police Interceptors on Channel 5!). I've kept my licence clean for 30 years - don't want to spoil it just for fancy plates!