Trailer refresh.

I reckon any jobsworth having a poke about your trailer would have a very hard time proving that you built it from scratch and didnt "comprehensively repair" it.

I searched for and found a trailer allready biult up back in the days when this was under less regulations. Dunno if this is so in UK as well, or if you can find such gems used or abused and cheap like I did. Took me a few months to find it, but when it appeared it was 5 minutes to drive to collect.
I used a trailer with the right measurements and weight for my project.
It still is home made as the registration says.
Dunno if this also applies in the UK but i do not think i could have done this to a type approved trailer from a large manufacturer as their construction is approved as is when new.

If it's a repair or refurbish you are not changing its design in any way, so just keep the registration information plate or the steelwork with any serial numbers etc. Chop away and replace like for like in gauge and size.

It becomes a "build" as soon as you impact the original design. You sort of know what the lines are and when crossed. So add an extra wheel set, make it longer / shorter or swap that 2inch box section for something else.

Of course this is all academic., It's impossible to police and more useless legislation that's easy to get around or fake. As you said, simply make sure it's safe and the old bill will leave you alone.