I have never been able to find any evidence that the so called outdated 85% rule was introduced by the UK caravan industry? I am not sure even if any of the clubs introduced it as it was only a recommendation for newbies and not even a guideline?
However some caravan insurers stipulate that the caravan should not weigh more than 95% of the car's MIRO or kerbweight.
I use the phrase "Caravan industry" in the widest sense, becasue it definitely did come from somewhere within the industry whether it was specifically the NCC, the Clubs or manufacturers is unclear.
There is an uncorroborated story that suggests in th e early days of caravanning , there were no guidances at all about the coupling or caravans to vehicle, but there were some decidedly unsuitable combinations which began to attract government attention.
It is said the Government called together representatives from the industry and users to discuss the issues. It is said the government laid down an ultimatum to the group, that unless the number of incidents reduced, they would bring in legislation and regulations to restrict the use of towing caravans. It is said the outcome was the the start of the "industry" advice. Despite my longstanding contacts within the industry , I been unable to find any formal documentation or admittance by any person or group to the formulation or ownership of the advice.
However I distinctly remember reading and old Caravan Club publication (with imperial weights and measures) which suggested using the the weight of the caravan divided by the Kerbweight of the car - subtly different but very close to the current advice. It was impractical as very few caravanners have any reasonable means of measuring their caravans weight to be able to do the calculation. It's simpler to assume it will be virtually at its MTPLM which is what the industry advice uses.
I cannot agree with the actual figures of 85 and 100% "they" (whoever "they" were) chose, as there is no public evidence of how they were chosen and what evidence was used to inform the decision, nor has there been any formal assessment been published of its efficacy and safety benefits . The only observable coincidence is the UK government has not applied any caravan specific restrictions over the longstanding trailer construction and use regulations.
I do agree with the concept of keeping caravans as small and light weight as possible, but I have no confidence that either 85 or 100% are the most preferable values non of them guarantee safety or necessarily legality - there are occasions where applying the advice without reference to the vehicles' specifications can lead to an illegal outfit.