I found this Australian website thought it might be of interest as it does mention the uk and interestingly apparently the Germans now have legislation in place that stipulates a caravan cannot exceed 0.8 times the weight of the tow vehicle ie 80%...
http://caravanandmotorhomebooks.com/making-caravans-stable/
http://caravanandmotorhomebooks.com/making-caravans-stable/