Oh dear, best put your tinfoil hat back on, the mind control rays are clearly getting to you
The insurance companies didn't write the law, though they are far from blameless in letting the various scams (whiplash, car hire, etc) get out of hand. And as I said, it matter not the slightest how good your driving is if someone else drives into you.
Here's a scenario - you are in a queue of traffic waiting at traffic lights, and someone runs into the back of you shunting you into the car in front. The owner/driver/occupants of the car in front claim against YOU
- so there's your £20k claim for whiplash plus courtesy car hire pus cost of repairs or write off. Naff all you can do about it, it wasn't your fault, but now your car gets looked over.
"Oh dear, this car isn't the one you told us you wanted insuring" will be the jist of what the insurance company will tell you.
So now you will have to foot the bill - or rather the insurance company will still pay out (they are legally obliged to I believe) and then come back to your to repay them. Like I say, hope you've got a good nest egg put aside to meet any claims.
But Plod will also take an interest, and on top of anything else, you are now facing a criminal conviction for driving without insurance.
OK, so you say you'll never stop that close, so you won't get shunted into the next car. Car driver behind claims on his insurance, a whole raft of stuff then swings into action and most likely his insurer will want to look at your car - unless of course you decide not to pursue the other party for stuffing the back end of your car in. Same result.
Or how about a pedestrian just jumps out in front of you - it happens. You prove it was his fault - hard to do unless you have a video camera running at the time. Even if you do, various people will have been looking at your car (plod will be particularly keen to see if it was roadworthy at the time).
Just remember, it's not the vehicle damage that costs - it's the personal injury awards. Cripple someone (as does happen, sadly) and you could be facing an award into millions for lifetime care and loss of earnings for that person.
In all these cases, and more. If it's spotted that what you have insured isn't what you are driving then your insurance in void. You've lied in order to get that contract, and the other party is not bound by it. So the only safe way is to tell the insurance company everything - if they want to be silly about it (as some do) then switch companies. All insurers have a certain target market in mind. For many of them that means steering clear of modifications - even if because they are too damn ignorant to tell the difference between something benign like a gas conversion and something that vastly increases risk such as some boy racer converting their hatchback with 3 times the power and no handling, brakes, or driving ability to go with it.
I don't care if you agree with that or not - from your attitude I assume you don't. But that is the law, if you don't like it, take it up with your MP.
Speaking personally, it's complete sh*t for brains people like you that are the reason we have mandatory third party insurance for cars. Way back when cars were still a novelty for the rich, there were already cases where drivers/owners couldn't meet the cost of damages awards - so it was made a legal requirement to have proper insurance (or to post a bond of what was back then a lot of dosh before you could self insure).
It's clear from the efforts that have to be taken to catch the criminals that a great many people would join you in the "will never happen to me" brigade and not bother with insurance. A lot don't anyway, and it's thanks to them that we have other laws that make life inconvenient (eg having to produce documents to get a new number plate) for the majority of us who are law abiding.