From Manchester to Glasgow, I'd imagine that journeys would be faster. Why? Because Pendolinos can casade down and be used on that route - which won't use HS2...Sorry if it's been covered already, but what's this about?
https://www.lbc.co.uk/radio/presenters/iain-dale/hs2-journey-from-manchester-to-glasgow-slower/
Certainly it has an effect for London to Scotland services, but the slower journey on the bits where only tilters can do 125mph north of Golborne would give a negative difference roughly an order of magnitude smaller than the positive difference being able to go up to 248mph south of Golborne does.
It's for want of a nail type stuff.
To a large extent, yes. And the higher speeds have hijacked the main need for it (capacity) in the public psyche - leading to stuff like the complaint about it that Dougal links to and the bogus myths about 'only saves 10 minutes'. But, at the same time, the Ultra High Speeds kill those stories dead when actual facts are examined, whereas some might still have a bit of life in them with 'only' 300km/h.The ultra high speeds were just (if you'll pardon my language) political willy-waving.
And the Ultra High Speeds don't cost much more than regular High Speeds, which in turn don't cost much more than Classic 201km/h Speeds.