You can use your smartcard for single, return and season train tickets on the South Western Railway network and most journeys across Britain, but check using the National Rail smart map before you buy your ticket.
If you have a zones 1-6 London Travelcard added onto your smartcard, you have unlimited travel within London on the Underground, Overground, TfL Rail, Docklands Light Railway, buses, trams, and most National Rail services in London.