Well none of my body kit has cost extra to insure cos they see it as optional extras so they didnt charge me for them.
Im not complaining, just call them and find out, better to know then to invalidate your insurance!
Definately tell them - but make sure you are specific - it is a genuine Rover dealer fit accessory, (the ZR spoiler from 2001 - 2003 the one without the rib was available right from the start of 25 sales as an extra) don't tell them it's an MG one they most likely won't like it. I had all the bits (bar the rear apron which was never availble for 25s) and alloys that it hadn't had from new insured on my 25 said they were dealer fit options and it cost £15 per year, never did get around to putting the skirts and front splitter one but was nice to know I could without any comeback or hassle at all
I have a ZR, but when I got it I they asked about being different from standard, I just told them I had the 17" Straights instead of the 16" hairpins, explained they were a dealer fit option and sorted, it's on my insurance as an extras but they said they'd be no charge. Mine is now with Churchill.