All the insurance companies I have had dealings with in recent years have clause in their policy T&C's that has the effect of requiring the policyholder to inform the company of any material fact that might affect the company's liability in the event of claim. This includes any modifications to the the standard car and especially those that will affect its performance, handling, or value.
As far as I am aware, no current car manufacturer fits a tow bar as standard. They are all an optional extra device selected by the customer , and whether it is fitted by the manufacturer, dealer, it is modification that effectively enables the vehicle to be much bigger, heavier, slower and it requires additional skills to manage it safely. This must be reported to the insurer..
As other contributors have pointed out, its is exceptionally unusual for the fitting of a towbar to increase premiums, but at least they cannot then turn down a claim because they were unaware of the towbar.
The 85% guidance has no legal standing whatsoever. and an insurer under it third party statutory elements cannot impose a lower towing weight than the manufacturer's specification. If the insurer were unhappy about a particular policyholders proposal, they would not impose a restriction, they would refuse to issue the policy.