The MTPLM of a trailer is not its weight. it is a upper weight limit, which means for legal reasons a trailer should never exceed that limit it should always be less than it MTPLM. The MTPLM is the limit for the total weight of the trailer, which is not the same as its towed weight, so attempting to use an MTPLM figure as a means of invalidating a claim on the assertion the towed weight limit of the vehicle has been exceeded would be laughed out of court.Some insurance policies have a 95% limit. Probably all they need to do is check the MTPLM of the caravan against the kerbweight of the car and decline the claim. It does not matter if the vehicle was capable of towing a caravan heavier than the kerbweight of the towing car. It may cost the consumer a small fortune to fight a case which may have a 50/50 chance of success.
There would have to be credible evidence of measured weights exceeding their respective limits to enforce any limitation applied in a policy or for any prosecution for a weight related infringement.
Car insurance is issued on the basis of the vehicles declared specification. An insurer would have to have a very clearly argued reason to impose a lesser limit for insurance purposes.
But as ever it is always a good idea to check the cover and any limitations an insurance police has before you renew or take it out.