As there's a difference of opinions I've started a discussion on the uk mailing list in the hope of achieving some local community consensus.
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Not sure what you're asking re: legality. I assume that CUK's route will be perfectly legal to cycle, but it might create legal issues for OSM if the data source isn't compatible with our licence.
I will repeat the following points which others raised in the previous changeset:
a) Explicit permission does need to be obtained before importing CUK's GPX data into OSM. They would also need to confirm that no other body (eg. Ordnance Survey) holds copyright in that data.
b) It goes against a long-standing convention to map what's on the ground.
c) If it is to be added, this route is not part of the National Cycle Network, so should not be tagged ncn.
On a final note, the real value of the OSM database is the data we collect ourselves. Taking a third party's publicly available gpx file and importing it doesn't really add much.
Kind regards,
Adam

Hi Ryan,
Welcome to osm, we need more local mappers. You might want to look again at the tunnel. A tunnel goes underground, so the canal towpath isn't a tunnel.
Any problems just shout,
Adam

Hi Zamm,
Thanks for contributing to OSM.
I'm sure it is not what you intended but you've removed the school tags from the school grounds and building and turned both of these into residential landuse areas.
Happy to help if needed,
Adam (ACS1986)

Hi Jerry, I've fixed this one but for future reference it's 'Wetherspoon' (without an 'a').
Kind regards,
Adam

It's okay, we just need to be careful when copying data from other sources - if in doubt don't. But please don't let it put you off mapping the areas you know or visit. You can add anything you see or know to exist on the ground. We can also copy and trace from the maps and aerial imagery available as background layers when editing - all of these have been cleared for use.
As I say, if you need any help then feel free to ask.