What Car? guide to... towing - Maximum towing limit
Exceed it and you could invalidate your car's warranty or be deemed to be towing illegally.
Watch out for cars where the towing limit is lower than the kerbweight - your outfit may appear safe in terms of weight, but the car may not be designed to do so much pulling.
A few cars have special conditions for the towing limit, such as one passenger only, so check the handbook.