The duty of the proposer to make a fair presentation applies.
It doesn't any more. Since the insurance act, the only obligation on the insured is to answer all questions and requests for information honestly and truthfully.
It's here:-
https://www.abi.org.uk/data-and-res...e-consumer-insurance-act-means-for-customers/
In the context of the thread, unless a policy
actually says you must inform the insurance company of any building works at all whether diy or by others, it is irrelevant. It would only become relevant if the work was both faulty AND was demonstrably the cause of a loss, which of course, unless it was DIY, would open up a counter claim for negligence against the contractor. I go back to my own policy, which says that I must inform them of works over £100K - that's their threshold. They could have said £10K or £1K, but they have chosen not to.
Sorry to be pedantic, but it's written far too often that not getting certification voids insurance, whether it's electrical work or anything else, and it generally doesn't. As a long-time DIYer of quite substantial works I've checked this several times with my own insurer's policies.
If you think about it, if the Insurance co's invoked such an "any modifications" requirement they would be completely swamped with information and the DIY industry would be in the toilet.
I accept that there is a slightly different perspective with motor policies where "modifications" can become relevant even if they seem immaterial, but until insurers introduce such a "modification" clause in buildings insurance, I think we are OK