Vehicle manufacturers are reasonably sensible, if the EU sets a line for vehicles not being regulated by operators licences and tachographs to record drivers hours (3500kgs) then they build vehicles to fit in (3490kgs).
The trouble is that the regulations for Tachographs and drivers hours relate to the maximum vehicle train weight, typically 3500kgs. If you then attach a trailer to a 3500kgs van the trailer has only to weigh more than the nominal 10kgs the manufacturers allow and your driver needs to have a tachograph when the trailer is attached and needs to keep a record of the hours he works as well as drives for the rest of the time.
There are certain exceptions, relating mainly to the use for none commercial purposes, otherwise the penalties can be expensive.
Another side issue is that individuals who have taken their driving test more recently do not automatically obtain a licence for a vehicle over 3500kgs or to tow trailers at all.
If you need help with what the requirements actually are then Pete Dodd in the Motoring, Regulatory and Transport department at Nockolds will be happy to assist, they might not be as obvious as you think!