Just wanted to clarify, as I have seen friends get serious charges for ignoring this.
Trailer capacity is NOT done with the kerb weight of a car, it is done with the "Gross Vehicle Weight" - this is the amount the car/trailer "could" legally carry if fully loaded. For some confusing reason, the DVLA then refer to this as "Maximum Authorized Mass".
This means to determine if you are legal, you should take the GVW from the information plate/manual of the car, and add it to the one from the trailer. THEN if that is over 3.5t, you aren't legal on a newer licence.
To use the Audi Example above, a 2-3 year old A4 Quattro Estate has a GVW between 1.9 and 2.3t depending on spec and engine choice. Assuming a mid spec derv, at 2.0 tonnes, this means the trailer can't show more than 1.5t as its GVW on the plate, or you are driving illegally. In this example, that would mean the trailer had to be the smallest possible car transport trailer I could find, with a GVW of 1300kg, which can only carry a 900KG car.
It makes no odds if the whole train only weights 2.5 tonnes while in use, if the combination of the two vehicles/trailer are CAPABLE of over 3.5 tonnes plated, you get nicked!