10% for parties of 6+; menus say it may apply wherever there's table service
10%Nicholson's own terms state a discretionary 10% service charge is added for parties of 6 or more. Its menus put it differently, saying a discretionary 10% may be added wherever table service is offered, so smaller tables may also see it.
Source: Nicholson's's own published policy. Service charges in the UK are discretionary: you can always ask for one to be removed.