That seems to be the wrong way round, Methyl!
Unless, and until, Parliament creates a statute specifically prohibiting cash payments for a particular purpose (as, for example, applies to the prohibition of cash payments for scrap metal under the provisions of the Scrap Metal Dealers Act 2013) all such payments must, de facto, be legal.
If you're aware of a statute prohibiting the cash payment of wages from the start of the current tax year, I invite you to provide a link proving it. (I'll be amazed if you can!)