You can use eGates in the UK, same as me as a British citizen (as long as your passport's new enough, probably all of them by now) - that covers it doesn't it? Otherwise I'm not clear what 'pre-clearance' is skipping? Painless for me into Canada too.
No, they mean we actually clear US customs in Canada as non-US citizens.
It saves you the hassle of going through customs when you land at your destination.