HMRC have taken to naming and shaming those not paying the full national minimum wage.  If you are a sole trader, partnership or any kind of company with shares, or operating a social enterprise with a profit motive, then you cannot avoid NMW by classifying your workers as voluntary.  The latest rates (from 1.4.16) are:

Year	25 and over	21 to 24	18 to 20	Under 18	Apprentice
April 2016 (current rate)	£7.20	£6.70	£5.30	£3.87	£3.30