Wow! I'just scrolled through ombudsman decisions for insurance being voided due to modifications, there are literally hundreds and I havn't found ONE in favour of the insured party! Voided insurance and refused claims for everything from tinted windows, wiring changes, different alloys, tow bar, HID's, even where someone fitted carpet to his van FFS!
U'm food for thought here, it seems it has nothing to do with whether you know it's a mod and nothing to do with it affecting performance or safety, simply that if the insurance company terms are that they will not insure a modified car unless mods are declared regardless of type, then the ombudsman HAS to uphold their position that insurance was offered on a false declaration i.e owner said there were no mods.
ZHP, drilled discs, lip spoiler, led lamps, lowered, air filter or roof motor moved? Heads up! http://ombudsman-decisions.org.uk/