In the USA, the IRS is going to require credit card processors to report what they pay businesses. It's very easy to spot a business that doesn't report or under reports their cash transactions. It was easy to cheat on but when the average business in the sector makes 20% of their revenue from cash and a 5% profit on their revenue they stand out like a sore thumb if they get greedy.
I'd imagine that the British are doing the same thing. They are just looking for people that fall outside the a standard deviation or two for whatever sector they are in.