Funny you should mention that, usual, tax evasion, yes I would and infact I have and it was a bloody good job I did.
When I was training I had a part time crappy job, the boss was a total idiot and he was employing people and paying people cash in hand and I reported it as they were clearing more than me, when I was above board and we were desperate for cash. I got another job and told him in no uncertain terms what I was doing. Turns out he was committing tax evasion on a massive scale,(not just by paying people cash in hand) he was taking money from MY wages and others too who were above board, for tax and NI, but not giving it to the tax office, so it was a huge mess for me to sort out as well. Luckily I did report it otherwise I would never have found out my tax was being screwed over which could have landed me in deep trouble. Along with many others who according to the tax office had not had their contributions paid.
The fact is usual, no matter which way you dress it up, benefit fraud is a crime, as is doing stuff for cash in hand, chances are they are breaking several laws, benefit fraud, tax evasion (both by proxy for not declaring taxable income)and denying people their consumer rights under law in the case of a garage. Mst self employed people though will be sorting out the tax themselves, so nothing to report is there?
I'd never touch a garage that did "work off the books". Why foreit all that protection you have under the law if it goes tits up? However that does highlight another point tax evasion is much more of a problem that benefit fraud. What you highlighted is tax evasion basically.
It depends on your own moral compass I guess, mine is I won't be fucked over by people who think they can break several laws to save a few ££££ as all the rest of us end up paying. If you know lots of people who do that, that's your social circle and I understand why you wouldn't report it, I don't want to mix with people like that.
In the 2 circumstances I have reported people I have done so because the risks to me were personal if I went along with it, I was not covering their backs, no bloody way.