I found this passage from the link below.
Company directors
..... 22 The minimum wage does not apply to company directors
unless they have contracts that make them workers.
Company directors are office holders in common law and
can do work and be paid for it in that capacity. This is true
no matter what sort of work is done or how it is rewarded.
23 However, company directors who have employment
contracts will need to be paid the minimum wage. If a
company director is unsure whether he has entered into an
employment contract with his company he may wish to take
legal advice. .....
A Detailed Guide to the National Minimum Wage PL501 (PDF 320K) (This is a link to the Department of Trade and Industry's Web Site)
http://www.hmrc.gov.uk/nmw/nmwfaqs.htm