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Either it's the law or it isn't.
This is a bit of a silly rule outside of a courtroom/mid-arrest though isn't it? There are plenty of things that are legal but stupid to do. Such as going out and licking door handles, which AFAIK is also not banned by the coronavirus legislation. Or tax avoidance.
Things get banned when there are enough people doing them that it's perceived to be a problem.
Edit: yeah why did I bother to write this? Bored, it's Friday, sorry
The problem with "guidance" is it's just that. If the UK government wants people to exercise but stay near home then do what the French have done; pass a law which says no more than 1 hour and no more than 1km from home. Easy.
Complaining about and threatening people who choose to ignore something that isn't the law is perverse. Either it's the law or it isn't.