In Feb of this year, my annual full service, including MoT fee, came to a total of £329. I have a service plan contract for this, for which I pay just over £26/month. It does sound a lot, but if you consider the hourly rate charged by the franchised dealerships, this is about the norm.
Bear in mind that this hourly rate has nothing to do with what their mechanics are paid. Business rates, heating, lighting, property maintenance, company cost of wages and N.I. contributions, other supporting service department staff, any materials used in the servicing process, and even property depreciation, all have to be taken into account.