LOL
I think what whoever said that meant was that cash, while taxable, is also untraceable and so if you were so inclined you could just pocket it and not pay tax on that income, though that is illegal.
What I mean is, if you get paid by cheque, there will be a record of it when you pay it into the bank, and therefore the tax man will have a way of checking on it. Whereas if you get paid in cash, and then rather than pay that cash into the bank you just use it to buy your shopping etc, then theres no way the tax man can ever know you've had it.
Although having said that, if you get investigated and he looks into your affairs etc and it seems that you have earned like £10 a month or something (because all the rest is in undeclred cash) then obviously he's gonna know somethings up and probably stick you with an estimated bill.
If you're gonna do it, the trick is to be clever about it and put just enough through the books so that it looks ok, while at the same time not driving around in a ferrari.