Then you can bet that it's part of the policy that the policy owner has to inform the insurance company of any changes to the vehicle.
Spoke to an insurance assessor (won't say which company
) a few months back and he told me that the 1st thing they do when a car comes in is "go over it with a fine tooth comb" looking for modifications. He then gave an evil smirk as he told me about a BMW 5 series they had in which he'd found a K&N panel filter in - Invaild Policy - I tell ya they're as cold as traffic wardens these days. I only got speaking to him because he came running towards me to cockily tell me my policy was invalid due to my exhaust - to which my reply was "and that's why i'm not insured with you any more"