If you ddidn't pass your test before January 2007 you would be lucky to be able to pull a sooty van.
If this is the case then your total weight of car and trailer cannot exceed 3500kg and the trailer you are towing cannot exceed the laiden weight of the vehicle.
If you passed before you can pull anything upto the maximum train weight the car can handle, above this and it is then illegal. Im having a semi stab/educated estimate of 3500kg braked trailer but this is reduced by how much weight you are carrying in the vehicle.
This is to the best of my knowledge, i am a post Jan 97 caravan puller so had to look a little in to it.