Its only the same a plumbers electricians joiners etc who work for cash only, under cutting those trades paying tax & Ni yet as a nation we ( not me ) seem happy to pay these people.
I have not heard of any builder being prosecuted for trading for cash only.
There are quite a few prosecutions by HMRC but I haven't heard of any around here. Locally I have heard of other self-employed people getting collared, they seem to do a hit by a particular trade and area now and then.
There are fewer prosecutions than there should be because a deal is often made, there is no public benefit in bankrupting people or wasting large sums of money prosecuting when the issue can be resolved outside the court.
There aren't many sole traders who need to pay tax if their accounts are done right which makes it difficult to prosecute for anything other than failing to file a correct return.
As with many things, it up to the public to report matters, the days of public inspectors are almost gone.