Yes, probably a bit dodgy.
Does the box trailer already have a VIN welded or stamped into the draw bar?
Dept of Transport are great for getting money out of you for bugger all, apart from shuffling some paper. I had a trailer that was badly rusted, so I did some upgrading. I replaced the hitch, drawbar, the box (made it a 7x4 from a 6x4), the frame and the suspension... oh, and the wheels. All that ended up being original was the the chassis number and the number plate.