I know loads of people who declare their nanny is self employed so the tax isn't their problem. I try to explain the tax rules but they don't listen. It irks me because I pay my nanny a gross salary so I am paying her 25% more than they are. but at least I am upholding the law!
Regarding big business avoiding tax so they should leave cash in hand alone
Firstly, taking cash and not declaring it is tax evasion. That is a crime. I don't think crimes can be compared and that is a slippery slope.
Secondly, the big corporates you mention are avoiding tax. HMRC 'let them off' because they had no position in law. UK Tax Law is hugely complicated and HMRC need to spend time simplifying it. That's how you cut out loopholes! HMRC have actually introduced a few things in recent years which have cut down on avoidance opportunities. however, the UK's tax regime is a big attraction for businesses so it is important not to throw the baby out with the bath water.