For pay periods starting on or after 1 October 2009 tips, gratuities, service charges and cover charges will not count towards National Minimum Wage pay in any circumstances. This new treatment is outlined in HMRC's Information Leaflet E24 Tips, Gratuities, Service Charges and Troncs.
For pay reference periods which started prior to 1 October 2009, tips, gratuities, service charges and cover charges could count towards national minimum wage pay if they were paid by the employer to the worker via the employer's payroll. The right to be paid the national minimum wage is a statutory right regardless of any written or oral agreement between the employer and the worker.
Further details of this new treatment and other tax issues for employers and those responsible for arrangements to share tips, gratuities and service charges amongst employees in the catering and service industries is available here