Tbh when I was younger and first into the modifying car world I didn't declare my mods because I didn't know it would be an issue by not doing so
As I got more into it / grew older I realised that as the above said if you didn't and a clued up traffic officer stops you / questions you then checks with your insurance if they're declared or not you'd be right in the s**t
As it happened when I was in my Celica which didn't have a standard part on it pretty much I did get stopped and questioned about it, thankfully I had declared everything with a specialist insurer and the premium wasn't too bad so I was covered anyway
As mentioned, you'd be driving without insurance which is 6 points instantly I believe and a hefty fine
You'd then have a mark against your name and it will then cost you more for not declaring mods when you have to fill in on insurance forms if you have any points on your license / what for
If you're with a specialist insurer it won't cost much anyway to declare then so you may as well suck it up, phone them, find out how much then either don't fit them until your policy is up or pay the extra there and then if only for peace of mind