Generally refunds aren’t permitted by the train provider if you don’t cancel your train booking in advance of the outbound journey. This means that if you miss your train, your ticket is typically no longer valid for travel on a later train service, unless special circumstances apply, for example you have a flexible fare ticket or the train was significantly delayed.
On the day of departure, if you know that you’ll miss the train, reach out to the train provider as soon as possible, to see what alternative arrangements can be made and to ask them to hold your return trip.