Yeah, unfortunately don't expect with 1 years NCB that it'll be alot cheaper and they'll be totally fine with the mods.
I've done really well for myself this year, 18 2 years NCB (1 year + 1 year free which they accepted) on a 1.8 driver was £720ish. I'm hoping that next year when I'm a member of the Institute of Advanced Motorists, 19 and with 3 years NCB they'll insure me for about the same price with mods done.
Even if a mod is just small like a back box and doesn't change anything but the looks and the sound slightly, they'll still rape you at a young age due to it being associated with boy racers/chavs/people that drive their car fast and wrap them round a lamp post.
I still think it's unfair that mods like brake upgrades and strut braces can make you pay more.. Better brakes make you stop quicker! So you don't hit that tree! I can understand roll cages though.