Reservations in Great Britain are optional, except for night trains and Eurostar. The app sometimes says there are mandatory reservations for other trains but that is because of errors in the British data.

For domestic trains, you can make a reservation for free at a ticket window or via myaccount.gwr.com. You first have to create an account. (It weirdly doesn't accept spaces in the surname.). Then sign in, and click Bookings at the top. Then look for the “Make a seat/bike reservation“ box. It will look as if you need to select a fare, select any, it doesn't matter (do check 1st or standard=2nd class), continue and make the reservation. It says you won't be charged, and indeed you get the free stand-alone seat reservation without needing to enter any payment details. Even though it's GWR, this works for any route where seat reservations are available.