It's not academic because if they felt the mod caused you to be a higher risk they could refuse to pay out on the comprehensive/fire/theft part of the policy entirely, or only to make a pro-rata payout. They have to pay out the third party part by law, but can even sue you for the payout if they believe you tried to defraud them with a false declaration.
You completely missed the point I was making . Which is that declaring after the fact and then attempting to claim for any modifications is not possible anyway unless your policy covers new for old so how you would prove such factory fitted extras to an insurance company is academic and they wouldn't ask or alter their payout anyway ,because you will only get book value for the car . Even in a new for old situation , anything not on the invoice would not be paid out for / provided
Anyhow , yes if an insurer did think that you have attempted to defraud them with a false declaration they may well attempt to refuse you a payout , keep any premiums paid and attempt to recover any money already paid out under the policy . But if their belief is not found to be reasonable they would be unsuccessful in such attempts to breach their obligations under the contract .
The ABI has clear guidelines on this , and it would be unwise for an insurer to attempt to withhold a claim unless they had evidence which made it reasonable to believe there had been a fraudulent declaration . The ombudsman also applies the law in a reasonable way so wont just side with insurance companies unless they can demonstrate a strong case .
Everyone thinks that insurers can just void policies at the drop of a hat . Simply an untrue urban myth that does the rounds far too often . There are things you can do to hold insurance companies to account .
At the other end of the scale, many classic policies include an agreed value - as the calculated book value that insurers typically use will be fully deprecated.
A common "modification" on Fords of a certain age was the replacement of the crap carb fitted with an aftermarket fit which might not improve performance, but would mean it could actually be started. Still, it's not OE fit so needed to be marked on. However, I need not declare the factory fitted options of a radio, heated rear window and parcel shelf! How much research insurers would do into the standard features of a 1979 car, I do not know.
That is indeed the case with plenty of classic policies , but they also generally cover much more limited mileage than every day use cars . And it is understandable why they employ an agreed value principle . With more valuable stuff they can even insist on conditions reports etc as this can effect the value of a claim massively . There are lots of difficulties with insuring classics and it would not be a business for the faint hearted .