Let's explore this some more:
Category B vehicles may be coupled with a trailer up to 750kgs MAM (allowing a combined weight up to 4.25 tonnes MAM) or a trailer over 750kgs MAM provided the MAM of the trailer does not exceed the unladen weight of the towing vehicle, and the combination does not exceed 3.5 tonnes MAM.
Cat B vehicles are up to 3.5t, which is where the 4.25t comes from - 3.5t + 750kg
So that's the first bit, then there's the 'trailer over 750kg' bit, which causes confusion.
Let's outline the situation using my kit - I know what it weighs.
When could you tow a trailer over 750kg? Say I'm towing with my Passat, which is stated in at 1875kg (it's a fat-ass, there's no denying it!), so in the situation where you don't have a full towing license, you could tow up to 1625kg.
BUT
Let's look at the real-world situation. Going to the day on Monday I had the following:
1875kg Passat
60kg fuel
400kg trailer
1060kg Golf2
30kg fuel in Golf
50kg luggage
100kg tools
2 x blokes at 95kg
2 x blokes at 80kg
64kg spare wheels
Total: 3989kg
Even if you cut it down to just the cars, fuel and two blokes you're at 3585kg and that's with a car on the trailer weighing in just over a tonne, a really light trailer and two skinny blokes.
Plus, unless you know how to manoeuvre a trailer already I'd be very hesitant in hitching up a full-size car trailer - it's not like a little box trailer you can unhitch and hoick about if you get it wrong.