The small boxes are made to be under 3.5t laden so can be driven on anyones car licence. Be very careful though in buying as most will not carry two horses and still be within the legal weight.
I know a couple who took their newly purchased 'under' 3.5t to the weightbridge and found that it would have gone overweight with their fairly small horse,tack and themselves

Fortunately they were both older so were able to have the weight upgraded to 5.5t and so use it.