Your insurance won't be void by not declaring mods. You will still be covered in an accident, but the insurer may then come after you to recoup the costs back. I assume they would need grounds that the undeclared modifications contributed to the accident though?
Having worked in insurance I know ALL things that change from manufacturer spec are treated with a certain amount of eyebrow raising.
I don't mean from call centre operatives I mean from loss adjusters. These are the guys who you are likely to end up dealing with in a more than medium bump. Some of them had quite extensive mech backgrounds, one who worked where I was had been an ex rally car mechanic.
The main problem is that in the UK anyone can list "upgraded" parts.
There is no validation that these upgrades are remotely what they claim.
E.g Upgraded brake pads. Some are track focussed and require more heat to work effectively (opposite to that normal road pads on the track often overheat and they go crap/wear excessively). Unless your really pushing it on the road the upgraded pads CAN (read doesn't apply to all) be less effective than normal OEM pads.
This was one example I remember discussing with a loss adjuster.
I was being a smart ass arguing similar points how I was a lower risk driver for having a quite heavily modded car, he pointed out how every thing I said had risks.
We really should have a system like TUV so the parts are tested to a standard (German system)
whilst tying its sprung to mind 2 of the reasons they REALLY dislike roll cages, 1) they give an immediate warning you must be expecting to be in situations where you expect it would benefit you and 2) they have massive concerns it will be fitted badly, imagine the claim if part broke loose and killed a passenger