I asked the question to Buckman, becasue he has used this claim previously, and it it is clear from the subsequent replies that there is general lack of clarity on the matter.
I totally agree with Lutz's point about the imprecise value of kerbweight. For such a clause to be workable it must use values that cannot be challenged.
But the other factor which the replies so far have not apparently picked up on is which insurance does Buckmans comment relate to. He failed to define if it was the statutory tow vehicle policy or the trailer policy.
It is my understanding that (in the UK) when you take out a car policy, the policy has to cover the vehicle for all its standard specifications, So if the manufacturer specifies its maximum towed weight limit above the cars "kerbweight" then the policy has to cover that extent of use.
However in the UK there is no statutory seperate insurance cover requirement for trailers, the cover offered for trailers can be limited in ways determined by the insurance company, and some (though none I have used ) could stipulate a maximum weight ratio but for it to be manageable it would have to unambiguous as to its value.