I didn't think it was anything to do with that. I was under the impression that so long as the trailer (including load) weighs less then the towing vehicle you were okay. Therefore if you drive a van weighing 2t unladen, then the trailer is allowed to be 2t including load but no more then 5.5 tonne combined weight including load.
Oops
Edit: Me getting confused....
Category B vehicles may be coupled with a trailer up to 750kgs MAM (allowing a combined weight up to 4.25 tonnes MAM) or a trailer over 750kgs MAM provided the MAM of the trailer does not exceed the unladen weight of the towing vehicle, and the combination does not exceed 3.5 tonnes MAM.