GASAX
He was probably stopped because the trailer had the potential to carry more weight than the Peugeot was permitted to tow. It is a 4 wheel braked trailer which can probably carry 3+ tonnes. The car`s max towing weight is probably half of this. This is a common gotcha if the BiB decide to pull you. It makes no difference whether you are carrying a 200kg aircraft or a 2 tonne one.
Once pulled over a simple check would reveal that the vehicle was not insured. As someone who regularly tows horse trailers, it is a part of the law you need to be aware of.