The following was submitted to CLIC by Ian Willis in answer to an inquiry about the 85% beginners towing rule. It was generally thought that Ian’s words helped to clarify the situation as regards to towing in Great Britain.

85% (or even 100%) rules are not "rules" but guidelines which have no substance in law. They are designed to ensure that the outfit will be stable (all other factors being equal). Exceeding this figure, whilst not illegal in itself, COULD lead to prosecution if the driver was stopped because of (say) instability or after an accident as there is a requirement on the driver to ensure the safety of his vehicle in the Construction and use regulations.
Usually a manufacturer will quote a tow weight which is far greater than the mass of the vehicle. Generally this is based on the total train weight (car + caravan) which can be started on a 12% gradient.
Where this weight (or any other e.g. nose weight) is given as lower by the manufacturer the user ignores it at his peril. Insurance could well be void after an accident - worse, the illegal combination could cause the accident if the vehicle is unable to handle it.
The big problem we have in the UK is the traffic density, especially on Motorways, which has caravans in close proximity to both cars and large goods vehicles at speed. The vortexes encountered can cause trailers to wander if not loaded correctly and matched to the towing vehicle.

Towing Law. Information:---- The NTTA version | Towing law from Practical Caravan | Towing law notes