If HMRC really wanted to find undeclared cash payments they could. They have the power to search your bank records, phone records, social media etc.
That one text "I've left your cash just under the plantpot" could be enough to say right youve been taking cash and we think youve roughly taken this much over the years so here's your bill and then it's up to you to prove you haven't.
So yeh of course we all know self employed people have been pocketing cash for years but youve got to be super smart about not leaving a trace to the point is it worth it, even your phone can be tracking your movements and storing info you dont realise...depends how much you worry about looking over your shoulder i guess.
I prefer all bacs for a few reasons, i don't have to chase the cash, i don't have to knock the door or fiddle about with change. Just scrub, post a note and go.