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.
> London is its own kind of beast with the tube/overground
To do well in London you really have to know the city (or at least the areas around your start/end points).
Especially as you get towards the centre of London, there are lots of opportunities for "cheating" (i.e. getting out and walking is much more efficient than going to your actual destination station, or, worse, changing trains).
If you want to go that extra step, memorising the tube map (or at least the central areas) also helps because then you can consider your re-routing options based on the actual live status of the tube (I say "actual status" because London Underground ALWAYS lie about the actual status .... i.e. they will always declare "good service" until they really can't avoid admitting the truth, by which time the service is truly up the spout)
As a London resident I find the app Citymapper pretty invaluable for making public transport decisions, it makes good use of both “shortcuts” and current conditions on the line to give pretty accurate results. Worth downloading if you visit (it’s also available in other major cities but I don’t know if it works equally well).
That all being said, I can totally see daily commuting by train being the absolutely most miserable thing.