Thanks for posting, I'm on limited data and speed again today.
Here's my view, not everyone agrees with this.
A modern trailer requires a CoC, or IVA, A frame makers will give you bit of paper for the A frame, car manufacturers will give you a CoC for car, I've never heard of any one having a CoC which covers A framed trailer as a unit, I've also never heard of anyone getting a IVA, so whilst it's fine for .gov agency to say you can use it in UK, that doesn't cover you for travel in a country that doesn't allow that exemption.