Tradesmen try it on with cash, that's why he'll have said that. A friend of mine is one of these builders who is forever folding and starting different iterations of the same company for tax purposes, and prefers cash in hand to the point if he wants to buy something off the xbox store he'd rather go to the shop and buy a points top up card with cash than be a sane person and do it there and then with a card.
He was whinging to me once that he went to pay for his feckless lad's car and turned up with £10k in cash which unsurprisingly nobody was interested in taking off his hands. This was an outrage, as far as he was concerned. Why shouldn't they take his illicit tax free untraceable cash?
Also I know some people who were running fags and booze across the channel in lorries years ago as a large scale operation to beat the duty and sell for a profit, they got caught in the end but while the good times rolled people were getting tesco carrier bags with £5k in them.