I know someone who tried to be a smartarse and run a heavily modded car on a straight policy. They found out via a forum, cancelled his policy and pretty much blacklisted him in the insurance market as a fraudster.
Nobody is legally bound to insure you, I can see blacklisting being more common soon enough.
Whilst i can understand the rationale behind that i would suggest that will only cause more of a problem. There are enough people on the roads already with no insurance or paying for insurance that is void due to undeclared mods. It wouldn't stop them driving
I see the wider implications, but that's not the insurance company's problem, that belongs to the law.
What would be there problem is him wiping out some city banker in his prime with a few kids and the several million pounds they'd have to pay out.. Every insurance company has the right to sue the policy holder for the money they've paid in a 3rd party claim if they deem the terms of the agreement to have been knowlingly broken.
So, insurer pays out £5m. Insurer finds out you'd been telling porkies. Insurer sues customer and wins (there's been several high profile cases recently and a precedent has been set).
= Customer's life ruined. They'll take every penny you have and will ever earn.. all for the sake of £50 to declare X, Y or Z.