UK rail is truly depressing.
In Japan you can take a Shinkanzen from Tokyo to Hokkaido, 4.5 hours, 800km (appx 500 miiles) for 20,000 Yen (£108) first class or 11,000 Yen (£60) standard class. And that is walk-up fare for "right now", you can use an app to get advanced fares.
And I can assure you those 4.5 hours will be insanely comfortable compared to the UK. And you can pick up some very tasty food at the station, unlike the UK where you can only buy garbage.
In the UK I agree, I'd fly from London to Edinburgh in preference to the ghastly train. Assuming the London train even leaves the station and is not delayed or cancelled, as usual.