I have re-read this entire topic, because it seems contradictory. In the first instant in #1 Dusty references a UK Gov't web site " Emergebcy Alert" notification which at face value does warns ALL trailers over 750Kg may need to be "registered" before being towed in "most" European countries. The document does not include any differentiation between commercial or noncommercial purposes.
Fundamentally the provision it purports to represent is not new it was started on 25 October 2018 and the list of European countries where registration is required was added in 2019.
So what has created the furore now? As I have never towed abroad I had never looked into the matter, but I have observed many British caravanners hitching up to journey to parts of Europe and they have never mentioned having to obtain a registration document.
Only in #14 Otherclive makes reference to
I suspect the mention of German police fining someone was solely a mix up in the post Brexit heat. Be nice to know when it occurred.
Despite googling "german police fine british driver for not having his trailer registered" the only reference was another forum where there was a hear-say reference to the incident, but nothing even remotely concrete.
I totally agree, with Otherclive and I'd like to know the reason the police were questioning it. I speculate there was some other reason why the police were involved, for example a collision or suspicion of stolen property, or even contraband.
In the forum there have been questions about formal registrations of UK caravans, but these have usually been in relation to the purchase or import of a UK built caravan into other countries where there may be import taxes and insurance and proof of ownership issues that might arise. and in some countries trailers do need their own registration plate.
In #17 AndrewAventur reports he has tried to register his caravan, but the online system told him it wasn't necessary.
This poses a dilemma where separate sections of the Gov't website are clearly at odds with one another.
Sadly no one on this forum knows the answer.