Then they are idiots. It is exactly the same as reservations for any open ticket.
Firstly you don’t need a reservation, they are not compulsory for trains in UK.
However if you want one the website for train operator https://www.gwr.com/ allows you to book them for free if you make an account.
Hello. Thank you. I Have made a reservation through GWR.
On the day you just show them the reservation and the QR code on the digital pass?
Thanks
On the day enter the trip into your pass and show the “ticket” with QR code the pass generates.
Nobody will look for your reservation under normal circumstances, just sit in the seat you have been allocated which will be marked on the displays above the seat..