All I can find at the moment is this from the camping and caravan club website, but will look into it further as I am on the committee ..
The Camping and Caravanning Club, like many others in the industry, recommends you only tow a caravan that weighs 85 per cent or less of your car’s kerbweight. Those who are experienced at towing may go up to 100 per cent of the car’s kerbweight, but no-one should tow a caravan that is heavier than the towing limit of the vehicle it’s behind. Legally, if you add together the Maximum Authorised Mass (MAM) of your car and the MTPLM of your caravan, the total weight must not exceed the GTW of the car.