It would have been a 15% discount for my dad (VAT was 17.5%), and probably a slightly larger saving for the tradesman, although they have a higher penalty if somehow caught. I think it was usually done for small jobs, like minor plumbing fixes.
I have no idea how common this was in the 1990s, or how common it is now.
Someone else can come behind me and cite the court case/credit card guidance
It's generally illegal, however, for a business to evade taxes by failing to report the full value of cash sales. It's also legally dubious as a customer to knowingly help facilitate such transactions (e.g.: anti-laundering laws)... but I don't think it's common to catch heat for this if you're an otherwise law-abiding person.