Fitzy From what I understand, owners of small businesses (which often don't have shareholders) can leave money in the business rather than take an income. They then take dividends or pay into pension schemes,so avoid income tax and NICs.
Ireally am not an accountant, but I did do some investigation of my ex-husband's accounts. He was claiming that he had no income, so I never received any maintenance for the children. I didn't believe him, because he has frequent holidays and has a generally comfortable life. He also owns millions of pounds of property.
It turned out that all the profits were being kept in the company (some offshore) which owns the property and his partner/wife and mother were being paid an income, which they gave to my ex-husband. The company apparently pays some corporation tax, but at a much lower rate than combined income tax/NICs. It's all perfectly legal and there was nothing I could do about it.
It might turn out to be the only good thing about Brexit from my point of view, because he was intending to become non-dom and live in his house in Spain. I hope his plans have been scuppered. Ha ha!
I expect there are other ruses too. HMRC probably know about them, but there's nothing they can do either, unless laws change.