However if you do not follow the guidance you could well be guilty of a crime under the legislation as a court would take the guidance into account in assessing the reasonableness of your excuse.
I think the analogy would be to the Highway code:-
https://www.gov.uk/guidance/the-highway-code/introduction"
Many of the rules in the Code are legal requirements, and if you disobey these rules you are committing a criminal offence. You may be fined, given penalty points on your licence or be disqualified from driving. In the most serious cases you may be sent to prison. Such rules are identified by the use of the words ‘MUST/MUST NOT’. In addition, the rule includes an abbreviated reference to the legislation which creates the offence. See an explanation of the abbreviations.
Although failure to comply with the other rules of the Code will not, in itself, cause a person to be prosecuted, The Highway Code may be used in evidence in any court proceedings under the Traffic Acts (see The road user and the law) to establish liability. This includes rules which use advisory wording such as ‘should/should not’ or ‘do/do not’.
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The second paragraph is key (this was an analogy remember...)
The argument that you may run foul of is the interpretation of "reasonable excuse". A specific police officer / court / judge may decide that going out cycling once per day is "reasonable" but going out for a second/third/fourth time is not. If you got out more than once you run the risk of someone's interpretation landing you in trouble.
Take the person who recently got done for driving from Nottingham to London and back "to buy bread". The speeding fine was obvious, but they were also fined for an unnecessary journey despite driving not being banned by any of the laws:-
https://www.itv.com/news/2020-04-10/how-do-the-coronavirus-lockdown-rules-differ-across-the-uk/"
Can I go for a drive?
...
But none of the laws in force in any part of the UK address the use of cars or vehicles at all and do not forbid members of the public from using their cars to “go for a drive” or travel to a location by car to exercise.
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Also remember that people do get arrested and convicted incorrectly, c.f. Daniel Cadden here. Being eventually cleared upon appeal is all well and good but it creates a huge amount of angst, stress and trouble along the way.
Finally be aware that the law in Wales is specific about only permitting exercise "no more than once a day".