Matching the Trailer with the Towing Vehicle

It is important that the vehicle you use to pull your trailer is adequate for the job

• Check that the engine is large enough to tow the trailer and load.
• Check that the brakes are powerful enough to stop the vehicle and trailer safely.
• Check that the Trailer Gross Weight does not exceed the Towing Capacity of the Towing vehicle.

The addition of a loaded trailer to a vehicle will inevitably have a very serious effect on the vehicle's performance. Starting, particularly on hills, can be much more laboured; stopping can take longer distances; cornering and negotiating sharp bends requires extra care.

Consider all these things very carefully when choosing and loading (and towing) your trailer. 
The paragraphs which follow, refer to the data that is relevant to your choice.  See "The Law"

The most important check is the vehicle manufacturer's recommended towing limit, which should be in  vehicle manufacturer's handbook and on the VIN plate on the chassis.

A good rule of thumb, for safety and stability, when towing a caravan, is the 85% figure recommended for caravans by the Caravan Club. This suggests that you should not tow a caravan that weighs more than 85% of the towing vehicle's kerb weight. (as long as 85% does not exceed the vehicle manufacturer's recommended towing limit. (The kerb weight is defined as the weight of the vehicle plus a full tank of petrol and 75kg (for the driver and luggage).)

Police Forces  use the manufacturer's recommended towing limit as their guide. Under no  circumstances should the vehicle's gross train weight be exceeded. You should also refer to limitations on overall length, details can be found in this guide that deal with trailer dimensions.