If a member of staff has employment or worker status under employment law then they are entitled to receive at least the national minimum wage (NMW) for each hour that they work in each pay period (which will be a month if they are paid monthly) and you cannot contract out of this right.
Whilst an employee may say they agree not to receive any pay for a prescribed period of time, if they later change their mind, for example, due to a breakdown in relationship, they may still be able to pursue a claim in relation to this underpayment of NMW. Such action would likely succeed and back pay can be up to 7 years, as well as fines for non-compliance is HMRC are informed.
HMRC can send a notice of arrears, issue a penalty notice and/or take employers to court. The maximum fine is £30,000 per worker and employers can also be named publicly and banned from being a company director for up to 15 years.
The NMW rates increase each April, but they are currently £8.91 for employees that are 23 or over.