What is law regarding casual labour?

Truli Asked: What is law regarding casual labour?

As in having someone to work for you for a couple of pounds very very occasionally. Is it illegal to just give them some cash in hand or is that only possible for esmall businesses/ sole traders ec NOT for Limited Companies.
Also if they have a second job who decides which is subject regular tax and which should be BR? Does it depend on the amount they earn?

ANSWERS FROM UK ONLY AND PLEASE ONLY ANSWER IF YOU HAVE A GENUINE KNOWLEDGE OF THE SUBJECT – CHEERS