Franchise TOCs have bid to operate services prescribed by the regulator, with either a subsidy paid by the government or a premium to be paid to the government. They must operate those services, and make certain commitments with regards punctuality, train refurbishments, investment in stations and other service matters. Most franchises consist of a mix of services from busy profitable commuter and intercity routes to quiet rural backwater routes- the TOC must run them all
Open Access operators apply to run individual routes. They identify markets not served by the franchises that they believe will be profitable. They will be awarded rights to operate trains over those routes at particular times, and pay a track access charge to do so. They don't actually have to run all those times- though I guess when their rights are up for renewal, any slots never used will be vulnerable. As I understand it, WSMR have oft changed their service level. The only penalty to not having a service is lack of income. They can withdraw a service without asking the regulator, and if the company is in financial trouble, the service will not be saved.