So I don't think there is any ambiguity.
If I have > 3.5T I need to register it - but very unlikely for virtually anyone on here. Certainly not routinely transporting back and forth.
More likely there are people like Pikey who do a once or twice a year trip to the EU with a <3.5T and > 750kg with their own boat etc. The "is this commercial" question must crop up enough for other things - tacho etc for certain things. Your insurance would also be for non-commercial use...
So that leaves the third case - you have a 750 - 3.5T trailer and want it commercially moved. (Probably not urgent). No sensible commercial mover will just turn up and hook up my trailer to take it to Europe. You'll want to do due diligence. You'll want to know its well maintained etc, legal. And I'd expect the commercial guys to know what that requires. Not "we don't know but we advise". Not "its the owners responsibility" - because rest assured if they are tying to explain to the Italian Police while I'm sat beside the pool - it wont be my problem! As for 'is it commercial' - dead simple... ...if I'm paying you to move it - its commercial even if its my trailer.
If I pay you to drive me in my car - its commercial.
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