<Devils advocate mode /on> I think the problem with not declaring is that they then leave themselves open to a higher risk. Like why you are supposed to notify them if say your car is broken into, even if you don't claim and fix it yourself because that means the current risk value of your area isn't considered and so your premium wouldn't accurately reflect the risk. You are therefore 'under insured'.
The other thing is trusting Jo Public to both recognise and know what constitutes 'a modification' and how the risk may then change. Whilst fitting a tow bar may increase the risk of something happening to the vehicle *because* there is something being towed (even if only damaging your own car when reversing a small trailer etc) but their stats may show that on average, people who regularly tow things generally have fewer claims than those who don't.
Like adding a second driver to a policy often reduces the premium because they consider both drivers will drive more carefully because they don't want to risk it for the other driver.
I remember when lads would say not report the modification of fitting different wheels (often with spacers) and / or smaller steering wheels, potentially making the vehicle more dangerous and hoping they could change them back before the
insurance assessor checked it over after an accident.
The last thing is that many things are going 'template' these days where say a broker will go to an underwriter and propose a package ...
insurance, green card, breakdown, hire car etc and anything that doesn't fit 'std' (like my kitcar) can't be covered by this package because there may be NO allowance for 'any' modification in their 'nope' list. <da mode /off>
They can be greedy b's though. WE had a car written off whilst it was parked outside our house and the lorry who did it just drove off. We had Fully Comp and NCB protection and they paid out (£350) but, because they weren't able to claim off anyone to recoup their costs, it went down as a 'blame claim'? It seems they don't want to take any risks themselves. ;-(