By knowingly paying a tradesperson in cash without a VAT receipt for the sole purpose of getting a cheaper quote by avoiding VAT it is the person paying that is effectively not paying the VAT.
But the tradesperson being paid in cash might also miss that job from their annual return. My brother-in-law is a carpet fitter. He pays an accountant to prepare his tax return in a way that doesn't arouse the suspicion of HMRC but substantially reduces his tax bill. Anything paid in cash makes that process easier.
(there are other issues with my brother-in-law which are probably not for a public forum)
But the tradesperson being paid in cash might also miss that job from their annual return. My brother-in-law is a carpet fitter. He pays an accountant to prepare his tax return in a way that doesn't arouse the suspicion of HMRC but substantially reduces his tax bill. Anything paid in cash makes that process easier.
(there are other issues with my brother-in-law which are probably not for a public forum)