There is an excellent Standard Note from the Library, SN00697 which will give you the latest information. This note provides an overview of the application of national minimum wage (NMW) law to persons working in a voluntary capacity.
Only persons defined in law as “workers” are entitled to be paid the NMW. In order to fall within the definition, a person must have a contractual relationship with his employer. An unpaid volunteer or intern without any contractual obligations will be ineligible for the NMW. However, contracts may under certain circumstances be implied through conduct.