The new laws came in 04/12/2011 but you can still tow for business behind a "Dual purpose" vehicle without an "O" licence as long as you have a Tachograph fitted.
If you are towing behind a van you need to have "O" licence inspection records for that van and the inspection records for the trailer and if you kept the trailer only for occasional use it simply is not worth doing - If you have an "O" licence and the work for it you need a 7.5 tonner - but then I think you knew that.
We have a 12' x 5' x 7' 1.8 tonne payload box trailer with low loading ramp / barn doors which can be used in place of a tail lift but can now only use it behind the Ford Ranger.
When we were using it behind a Renault Master the MAM was 5.5 tonnes and the legal payload was a very useful 3.3 tonnes.