Cardiff - Hereford is £15.50 to upgrade on the day. You’ll get free tea & coffee/juice, and some snacks, can’t remember the specifics of the snacks! The main food is available to purchase from the menu. Other perks are the obviously more spacious 2+1 layout, large seats and more privacy. The ex GC sets are a little neater and tidier, the old LNER ones are definitely showing their age a bit more now but the layout is the same.
Diagram wise I think it’s hard to tell at the moment with certain sets and locos being out of use for various reasons.
@sd0733 is normally able to provide some provisional workings a bit closer to the time.