When I sold my last property, there was a term in the contract that
said "there are no terms of contract, apart from the terms written
here" - so anything I had said about the property was not a term of
sale. I did however fill in a form included with the contract (mostly
detailing what was and was not included in the sale) that contained
questions like "has there been any building work in the last 5 years
that falls within the scope of building control?"

So at that point you would be wise to come clean about the kitchen
rewiring being unapproved.

However (AIUI - IANAL) all the old work is just "condition of the
property" and carries no responsibility to disclose on your part.

In practice I have always been scrupulously honest with my buyers, and
it has been appreciated.