I find it interesting how you selectively left out of my insight and background as to why he would get covered in the case of an accident.Not knowing UK laws or OP's policy contract wording, I am reluctant to advise OP to act against the insurer's advice.
Even contracts aren't justification as to why an accident wouldn't be covered if unless the exhaust is the at-fault reason. This is an EASY case in court. Threaten litigation and the insurance reserves goes up further than the payout. OP gets covered without this going to court - this has nothing to do with US LAW, this has everything to do to how insurance works fundamentally.
OP is an adult and can decide for himself.