reality is this:
1. you can advance all sorts of arguments, reasons and explanations why your mods need not have been declared;
2. however, if you are claiming or if a third party is claiming against you, mods not being declared gives the insurance company an opportunity to reject your claim .. an opportunity which will be taken up if at all possible
3. you are then left with having to fight the insurance company .. you need to spend huge amounts of time and money to do this. Meanwhile, your bills are stacking up, and are your stress levels.
4. if the insurance company does not budge, you go to the insurance ombudsman or to Court, both of which takes a lot of time and effort.
5. yet months later, nothing is resolved.
6. and may never get resolved in your favour.
7. meanwhile, your car is rotting away, unrepaired, you have no transport and lots of stress
Is it worth the risk of all that aggravation to save a few pounds?
Fundamentally, insurance is a risk allocation game. If you don't tell the truth, it's not unreasonable for the insurance company to say no cover. Not declaring mods is, in effect, cheating.