An interesting question popped on a Facebook page this morning , i would have said no myself 
" Can anyone answer this ?, got asked today weather you can tow 3.5t trailer behind wagon without having it on your o license? "
If I had to take a guess I’d guess that yes you could tow it outside of the regs as the trailer itself does not fall within the regs. Similar I suppose to tractor units on their way to truck shows towing large caravans. But I’m no expert myself so would be interested too in seeing a definitive answer.