I couldn't think of anything worse than turning up at the station for a long distance train trip only to find my seat is not available and no alternative place to sit! When I travel on regional trains I am almost always on it for more than 4 hours. No way would I stand for that long. I would rather take a road coach or fly at least they allways have a seat for every passenger no matter the delays.
Reservations are available on all long-distance trains in the UK, albeit not compulsory. You can make a reservation when you book your ticket, at no extra charge (and of course many of the cheaper tickets
require you to make a reservation for a specific train). If you get on the train and someone is sitting in your reserved seat then 99 times out of 10 they will be happy to move if you ask politely, I've never had any problem on that score. With many key routes running every half hour or better, the prospect of losing all that flexibility by being required to make a reservation in advance would be a huge disincentive to travel by train – if the train only runs a few times a day (or in the case of some Australian trains, once or twice a week!) then making a reservation is much less restrictive because you know for sure which train you'll be travelling on, but UK railways operate in a very different context!
Also, a note on terminology - in the UK, we would generally use "regional train" to describe one a step down from "intercity train". Regional trains are for making middle-distance journeys, typically 1 to 2 hours between towns and cities on lines that are away from the main intercity axes – and while some
trains might make longer journeys (the Liverpool to Norwich service clocks in at nearly 5½ hours) there will be very few
passengers who are riding the full length of the route.