In UK you do NOT need to make REServation all the time-except the 2 or 3 overnight trains to Cornwall and Scotland. You CAN if so wanted do it for free at any counter-in a few seconds. You do not need to fear that trains are ´full´ weeks or even monthes ahead-plus there are multiple trains between these 2-even more as 1/hr. IC-trains with REServation optional/possible have small screens that show what seats are REServed between which stops. It seems to me that most such trains have at least 1 car without any such REServ.
More info-also about the other countries is a little bit hidden in ´country info´ on main site.
In fact-from a recent visit to there- the major prob is more likely that your train requested weeks ahead is suddenly cancelled-staff shortages or the next strike.
No reservations are needed for day trains in GB - the app has incorrect information because the British operators (via RDG) are sending incorrect train information in the timetable data. You can however make reservations free of charge at a ticket office in GB or via GWR using these steps.