If you are travelling from an unmanned station without a ticket buying machine then you are allowed to wither buy a ticket with railcard discount either from the guard on the train or when you arrive at your destination/connection.
Most unmanned stations have ticket machines though.
Advanced fayres that are valid only on certain trains are always cheaper than flexible, on the day tickets.
Never use trainline as they charge a booking fee. www.nationalrail.co.uk/ have no fees and neither do the individual train comapnies website eg Virgin, East Midlands etc