If you have undeclared modifications then you, frankly, deserve to have the claim not paid. Many of us pay extra to have our modifications applied to our insurance. If you can't afford the mods for insurance you shouldn't be modding them.
Some insurers will allow you to retrospectively pay, in the case of minor mods for example and exhaust or an air filter. Some don't.
But if you have bodykits, wheels, air ride, retrimmed bucket seats etc, then they'll just cover third party costs and you'll be left with a damaged vehicle.
Also if they do decide to cancel your claim you may be liable for the costs of the courtesy car and it won't be at the normal 45 quid a day you'd pay for such a car....more like 145 quid a day.