There seems to be a misconception here about declaring modifications, just remember that ignorance isn't an excuse. It is our responsibility to know what we are driving, this is more difficult with modified cars due to possibly having to check the whole car for anything that looks non OEM, but for non modified cars it's very easy to get a list of all optional extra's from any main dealer of that marque.
I've insured with the Admiral Group a number of times, very easy to work with, very easy to declare mods with, but of course they're not covered on a like for like basis, but you'll be legal. I've also had a claim many moons ago with Elephant which was completely painless. The car was insured for £1500 and the repair was £1250 and they never even mentioned writing it off thankfully.