Here in the UK, this practice is so common it has a name: "paying cash in hand". Its literal meaning is just paying cash for something, but its implied meaning is that the recipient won't declare it to the tax authority (HMRC), and you often get a discount for paying cash in hand (so I've heard, I've never actually been offered this myself). Of course we're talking about individual tradesmen here, like plumbers or handimen, not big or even medium businesses.
Being paid cash in hand is usually how most teenagers get their first job too. If govt. clamped down on this then you'd see a lot of youngsters struggling to get onto the market, and I believe that's why they let it slide.
I'm not sure I totally buy this argument - the logic being that teenagers are only employed because employers can get away with paying them less due to lack of tax. But there are already lower minimum wages for these ages, so it'd still be cheaper than hiring older employees. Plus a lot of these jobs would be part time, so could potentially result in an income lower than the personal allowance (currently £12.5k) and be tax-free anyway
Yep, I'm in the UK too, I'm familiar with the practice - my point is just that it doesn't seem like a legitimate argument to be anti-electronic payments
