Like others have said I have never come across such a towed weight restriction on a car policy, and I must say I find very unlikely, becasue the the regulations about car insurance make it clear the insurance must cover the car and all its specifications. An d that should include the maximum towed weight regardless of any percentage of kerbweight.
BUT as ever with things like insurance the owner should read and fully understand the the insurance policy and if there is any towed weight restriction that is tighter than the cars own manufacturers specification, if you want the policy to remain valid you must abide by the policies T&C's, Or change to a policy that does not artificially restrict the cars capability.