Taxi driver takings varies hugely. I was talking to a driver recently. Because of the excessive number of licences his authority handed out, I was the first passenger he'd had from the rank in an hour. Self employed owner driver. My fare was £4.50, so that's his income for the hour less fuel etc he probably was at a loss!
The same thing is reported to be happening with minicab drivers in London, due to that god-awful uber bunch. TfL do not restrict the number of minicab licenses dished out and (as anybody who has witnessed the driving of the high number of Toyota Priuses etc can tell you) the bar is not exactly set very high.
Conversely, proper taxis (hackney carriages) can earn a respectable living - just look at the places in London you see taxis on the driveways!
My grandparents always used to give the paperboy, milkman, postman, binmen a "Christmas Box" back in the day which I did think was a nice touch, but it's a tradition which I think has passed generally now.
Or, as alluded to, that people no longer have the individual connection.
When I was a child, only 20 years or so ago, I knew the milkman, postman, chap who ran the paper shop etc by name, and the bin men were regulars. Nowadays, there isn't any continuity (and some trades such as milkmen are far less widely seen than before, whilst newspapers may not come from Fred Blogg's corner shop, but the Tesco Express).
The phrase "cooking the books" comes to mind. A transaction that doesn't go through the till not wanted by card so there is no record of them, just like my local mechanic knocks 10% off the labour rate if I pay by cash and don't need an invoice for the labour, I.e. An invoice for supplying a part to diy fit rather than one showing the works been done. I never do this as I like a paper record of what's actually been done to my car, but I've been made aware by him such discount is available. He is a one man business working on his home driveway.
The morals surrounding tax evasion are interesting. If, for instance, a tradesman offers such a discount, it is usually obvious why. How many people would take the offer up?
Equally, on a wider note, say I repair my friend's car/laptop/phone on my day off and they give me £40 cash - now presumably to be above the board, I would need to report this income to the Inland Revenue.
But what if, instead of giving me £40 cash, they take me out to a nice restaurant for dinner which comes to £40? Both essentially the same benefit but one in a more liquid form. Neither is in pursuit of a commercial business - just a favour for a friend.
(Equally, if you give somebody a lift in a private car, I understand that they can give you petrol money without any problems but if they give you more than that, in consideration for your time, you should declare the income and have appropriate hire and reward insurance... so what if you give somebody a lift, they give you £5 although the petrol cost is somewhat smaller?)
And to go the full circle back to taxis, rules change in October (I believe) and they will mandate that all taxis must be fitted with chip&pin readers. So will we see a sort of reverse-tipping - "that's £31.80 on the meter mate, or £30 for cash"?
(All of this, of course, is somewhat irrelevant compared to the huge amount of tax evasion by large businesses, but it would not suit to have them held to account).