Having both traveled across the country and throughout Greater London on trains, it is actually really convenient. London is its own kind of beast with the tube/overground, but throughout the rest of the country its definitely pleasant. Going through London during peak hours, dealing with trains randomly stopping midway through a journey with a packed carriage, dealing with anti-social teens, or traveling in the middle of summer in the tube isn't super fun. But on the other hand, going through the countryside on a long distance train like going up to Scotland is great; I basically just watched the views go by for 4 or so hours.

That all being said, I can totally see daily commuting by train being the absolutely most miserable thing.