A UK registered vehicle has to display the appropriate national identification letters on the vehicle and any caravan or trailer you are towing. The approved pattern is oval with black letters on a white background and size (eg GB) at least 175mm by 115mm.
So for France: 
or Poland: 
, etc.
Even though FR is not in use as a country designator in the EU, I'd hate to have anything on my car that made people think I was a cheese-eating surrender monkey.