i wouldnt worry about logbooks/tax details myself but of course you should declare mods that matter but having worked in a main dealer bodyshop i do know nobody really checks

cars comes in for pricing and a list and price is made and pics are taken and emailed as proof of damage (if required) a workshop wants business so wont rock the boat by getting shirty with a stainless exhaust and prob wouldnt even know if that engine was std and even if they suspected they wont get a copy of you cover, just a yes or no to cover so maybe its declared, its rare to not have std for modded neways so it wouldnt matter in the number either

of course a big crash and some investigation will get you in trouble fast and if the cops impound you car you may have to prove you are insured, if they copper knew his stuff maybe he coudl instist insurance covered mods before it was released??? when people collected them when i was there we used to do the old 7day wonder checks of paperwork and take some cash tho