As long as the lorry has the correct towbar for the trailer and the driver has the correct licences then it is fine.
A carriage driving lady near me, drives a HGV lorry pulling her carriages in a trailer all over Europe.
I think its pretty standard for carriage drivers to have a trailer behind their horsebox to tow their carriage in: not sure what type of trailer though!
Yes and in France. Though they are often 9 horse boxes with a tag along on the back - often seen over here too, delivering and collecting racehorses and breeding stock.
I drive a 3.5 ton van towing my trailer with no problem, it actually tows better than my big 4x4. I must admit I would rather convert the van to a horsebox but then I would tow a caravan behind it for the shows we stay overnight at.
TBH if I had to have a trailer (and I can't stand the awful things) I'd rather tow with a van if at all possible, thinking of the weight distribution in a van I would *think* it would make a very safe towing vehicle?