My van is a six berth, and is constructed to carry 6 people belted. Two are lap belts on the backward facing dinette and the other four are three point, driver, passenger and the two front facing seats at the dinette.
However, I have been informed (and I cannot remember where it came from), that I could carry other passengers on the U shaped seating at the back of the van (no belts). If the passengers were, in the opinion of the police, making the vehicle 'unsafe' by their movements in the back then I could get cautioned (as the driver).
The wording I have used above was not the exact wording I were told/advised...it was a while ago!