You need a licence to own a multiplex. Ofcom advertises licences for which interested parties can bid. Once the multiplex licence has been awarded by Ofcom, the new owner will seek out services to broadcast on the multiplex.
There are two national commercial multiplex owners (Digital One and Sound Digital). The BBC has a separate national multiplex for its services. There are local multiplexes around the country, each broadcasting an average of seven services, plus the local BBC station.