Just a word of warning, don't take this as a gospel as I'm myself not 100% sure on this, but from what I understand it doesn't matter what you have on the trailer, what the police are actually interested in is the gross weight of both the trailer and the towing vehicle. In case of the Transit the gross weight (fully loaded) will already be close to 3 tonnes, so you could only tow a very small trailer, certainly not one that would take a car. I've looked into this before and basically if you want to be able to legally tow a car you must have B + E and a car big enough for it to have a kerb weight higher than the gross weight of the trailer you will be using (yet another rule which kinda makes sense).