https://en.wikipedia.org/wiki/Freedom_to_roam
You can, in general, walk/cycle to most places in Scotland although there are fairly sensible conditions:
https://www.scotways.com/faq/law-on-statutory-access-rights
https://www.outdooraccess-scotland.scot/
I'm in the process of walking the Munros and I'm careful to check with landowners about access during the deer stalking season - I've never had a landowner be anything other than completely helpful.
NB Walkhighlands is a superb site covering walks throughout Scotland, not just the Highlands:
https://www.walkhighlands.co.uk/
Heritage Paths is also good:
That's not strictly true for England. Although it's not illegal to trespass (outside of power stations, military bases and ports) unless you cause damage while doing so.
I’m wondering if similar projects would work outside of the UK. As far as I know, the UK is pretty unique in the fact that basically all land is freely accessible by anyone, except when explicitly forbidden?
For example here in the Netherlands, basically anything that isn’t a public street is off limits, except when the owner of the land explicitly grants access.