It's not an instruction, it's a recommendation. The only time, by law, that you have to obey the highway code is when it says 'must'
"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’."
"You should wear a helmet."
I'm not expressing an opinion, just clarifying the facts.
@freddo started
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"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’."
"You should wear a helmet."
I'm not expressing an opinion, just clarifying the facts.