Well I personally disagree 100%. Maybe in the UK that’s how things work but I like to think in Ireland they aren’t so sinister trying to catch you out. Through no fault of my own I have had to use my insurance on my GTi. Mapped and pedal box. This was never disclosed to the insurance company and was never an issue for either claims. (Though the map and pedal box was noticed both times) each to their own tho. I just see it as been a massive waste of money for already excessive premiums 
FWIW, Brentacre and probably other companies only charge for mods that increase the engine power. You declare everything, but if it's just cosmetic, then they just go "yeah, ok" and add it to the policy. You might wonder what the point is - it is so they can correctly restore your car if you make a claim.
E.g. If you have aftermarket alloys, some companies will give you stock alloys back if you are in an accident and they get ruined. Brentacre, if you've declared them, will replace your aftermarket wheels like-for-like