Failure by the builder to declare income to HMRC would be a criminal offence under Section 3 of the Fraud Act 2006. ('Fraud by failing to disclose information').
Further, Section 1(1) of the Criminal Law Act 1977 states: “If a person agrees with any other person or persons that a course of conduct shall be pursued which, if the agreement is carried out in accordance with their intentions, either will necessarily amount to or involve the commission of any offence or offences by one or more of the parties to the agreement, or would do so but for the existence of facts which render the commission of the offence or any of the offences impossible, he is guilty of conspiracy to commit the offence or offences in question.” (There's also a common law offence of 'conspiracy to defraud' which can be used as an alternative charge, as per the provisions of Section 12 of the Criminal Justice Act 1987).
So, while it rarely happens in practice, it is possible that a customer (who knowingly pays a lower price on the basis that a trader won't then declare that income to HMRC) can be prosecuted, alongside the trader, for committing a criminal offence.