From the point of view of being an Assistant Underwriter for a major insurer i can tell you that any and every modification from standard specification would class as a matterial fact and have to be disclosed to the insurer. Had this not been done then in the even of a claim your cover could be reduced to that of Road Traffic Act only cover (and that's even less that Third Party Only - it's the minimum level of cover legally required in the UK). Alternatively, your insurers could charge you a backdated additional premium for the modifications and deduct this from the amount of any settlement payment to you following a claim - worst case scenario, your car gets written off, you lose your car, claim, end up with no car and actually owing the insurers money (the amount of the additional premium for a modified car could easily exceed the value of the car).
It's got to be best to get this sorted in the 1st place, after all some companies like young drivers, some like mods, some like both, and some like none. It's best to be with the one that suits you form the start rather than try and change mid term when they can also sting you with cancellation charges.
Also, don't forget that if your insurance company cancels your policy because you haven't disclosed a material fact then that cancellation becomes a material fact in itself. Meaning that you have to tell your next insurance company that you have had a policy cancelled by a previous insurer - and most companies won't then insure you. If you don't tell them then this is another reason for them not to pay out / add huge additional premiums / cancel your policy.