You can generally reserve seats for free before you pay, but seats are allocated based on seating capacity and cannot be guaranteed.
Some train operators do not allow passengers to reserve seats. If you need more information, please contact the train company which is included in your confirmation email. For information about all train companies, you can visit the National Rail website (https://www.nationalrail.co.uk/). Even if no seats have been reserved for you, your ticket will still be valid and you can sit in any empty seat. Once you complete payment and your tickets have been issued, you cannot reserve or change your seat.