Whether you can travel with a different train company to the one you hold a reservation with depends on the terms and conditions of the particular ticket you purchased.
Prior to your departure you should check the terms and conditions of your ticket online or with a member of staff at the station before you board the train.
The train company you can travel with will usually be printed on your ticket. If your ticket says “Any available”, you may travel by any reasonable route to reach your destination, using any train company running services.