I'd like to know where this plate come from. I suspect that it could be a custom plate from Luxembourg, but I'm not sure because this is not the usual format for european plates (color, no country letter).
I know this may not be the exact place to ask, but that's the most accurate I've found.
Plates from the channel island of Guernsey are digits only. They follow the UK size and colouring but white on black is still commonly seen as well as the reflective black on white/yellow.
The country plate on the back of the car will be GBG.
Your idea of Luxembourg doesn't apply, their plates are always black on yellow and have slightly smaller characters than the UK/GBG plates.
