I think this is the legislation, but I'll stand to be corrected. A word search on "trailer" elicits about 150 matches, but "individual vehicle approval" only about 50, so may be a better place to start.
On one of the links I posted earlier, there is reference to trailers built US and Canadian standards also being accepted, provided they were built over there, approved and subsequently imported, as their standards, whilst different, are considered to be sufficiently rigorous to ensure safety. This again relates to it being a UN requirement/recommendation, rather than just the EU getting all bureaucratic/authoritarian.
I'm sure Plasma is aware, but not that far from him is a DVSA weighbridge, where I've regularly seen vehicles in for what appears to be an inspection as well as a weighing; I was always a bit nervous going past on the way to a rally with my Harvey Frost Towboy and a lot of ironmongery in the back of the Landrover.

Personally I might just pick up a plate off ebay just to keep in stock should I ever need one. 