For UK travel it depends a bit I think - a daily commuter's experience will be a lot different to that of somebody who is making off-peak leisure trips. For the latter my experience is that there's rarely crowding (especially in these pandemic times!) and that advance purchase (fixed to a specific train) tickets are often pretty good value.

(Writing this on a train from Exeter to Dawlish. The sleeper train from London to Penzance is good if you like that kind of thing.)