Tax man generally go for the low hanging fruit.
I was investigated over a few hundred, my mate over a few thousand.
I know of an accountant that was investigated over an unspecified amount, he took it to tribunal with a barrister representing him. Turned out they wanted access to his personal accounts, which they weren't entitled due to enquiries were into his business. They tribunal found partly in favour of the revenue and also the accountant. He had to provide bank account statements but could redact all personal expenses. Seen as they were personal accounts it was literally pages of blacked out lines. There was one error, came to missed tax of a few quid