well i had my corsa from 17 - 20 and I declared alloys, tints, back box, induction kit and what they call decorative exterior which i passed it as stuff like upgraded lights grill etc. dint go up massively but when i chose suspension kit it wouldnt give me a quote so i just dint put that on. luckily though i never had an accident with the car. things like back box induction kit tints won't make it go up by much as it doesnt improve performance.
I was with elephant.co.uk with all my mods and was cheaper than a standard car on other insurers lol. Adrian flux are also good and may allow all your mods for cheap. Im with them now with my clio 172 with m ods decalred and it cheap. but i have heard they got bad customer service but im not to bothered.