I have a somewhat interesting story.
I had a Ibiza Cupra, I declared all of the modifications that came on it.
I swapped from Adrian Flux to Admiral after a house move, Adrian flux wanted to take my premium from £1000 a year to £2400 a year.
They then dropped it by £1000 when I kicked off but I still swapped to Admiral.
About half way through my Policy I'd had a call regarding a 'Dump Valve'
I informed them that I don't have a dump valve I have a re circulation valve and it was replaced by an aftermarket piece because the original had failed.
I was told that my insurance was invalid because they WOULD NOT COVER power modifications... I argued because I had remap declared, Exhaust system, Battery in the boot, Spacers, Huge Jetex cone filter and a Carbon fiber enclosure for aforementioned filter and all of these things were covered.
I was told there and then my insurance was invalid and not to drive the car until I'd sent them a signed receipt stating the modification had been removed.
I argued a few more times and eventually gave up.
So there I was... 300 miles from my house, About to catch a ferry to the isle of man with the Car club for a weekend away,
I 'replaced it'
But seriously, I despise these companies if it doesn't enhance the performance or alter the external looks to an extreme degree you shouldn't have to list it.
Its just another way for them to siphon money from you!