We recently returned from a month in the UK (Scotland, England) and traveled primarily by rail. We had purchased BritRail passes (8 travel days in a month) from ACPRail. Overall we found travel with the passes easy and had no major issues; we used the National Rail Enquiries app to check timetables, delays, etc., and sometimes resorted to individual lines' apps.
That said, how seating is (or isn't) managed varies between lines and imho overall is not great. My main takeaways for UK train travel only are (and your mileage may vary):
1) Do not pay your pass provider (ACPRail for us) for seat reservations, it's a ripoff as some trains don't actually reserve seats (see #2), and if they do you can do it for free at any station. I would only consider paying if I were boarding a train immediately after arrival in the country, didn't want to risk getting bumped off the train, and knew that the line would honor the reservations.
2) Know your lines' policies. Of the four lines we traveled on (ScotRail, Northern, Cross Country, South Western), only Cross Country allowed seat reservations and honored/enforced them. Northern and South Western both said "sit anywhere". ScotRail was still making reservations (false hope there lol) but we were told on the trains by an official to sit anywhere.
I couldn't tell if the move away from reservations on 3 of our 4 was due to staffing levels, they're tired of having their people argue with entitled s**ts taking the wrong seats, or what. And sure on some journeys they aren't necessary. I just found the lack of consistency and information rather disappointing as a tourist.
As for the seats themselves, if you do get reservations on a line still doing them, they are first identified by the car (ABCD...) which refers to position of the car in the direction of travel. For us only Cross Country clearly labeled the cars, on the main lines but not others. It was particularly confusing to us when we boarded a train in Birmingham that then went back out the way it came, so what we thought was the D car turned out to be A... maybe. In the bigger stations with automated platform signs would it kill them to indicate direction of travel where it might be ambiguous? I'm sure the locals all know what they're doing but for the uninitiated, as I said, at times confusing.
I had read a few horror stories about entitled peeps occupying seats and not giving them up without a fight, but that didn't happen to us. (Well, on one ScotRail train a gal was in our seats, but as we'd already heard sit anywhere, there was no point saying anything to her.)
Line summary:
-ScotRail, seat reservations, neither honored nor enforced
-Northern, no reservations, take any seat
-Cross Country, reservations, enforced. But...
Our international-sized travelpro carry-on suitcase didn't fit in overhead storage and end of carriage luggage area was quite small, which was a problem for us on one train when we had no option but to jam them in the seat with us. On one train we saw a car clearly marked on the outside that it had extra luggage storage place, but if you're busy trying to figure out which car to get on and only have 2 minutes, well, good luck with that.
-South Western, no reservations, take any seat.