Usually, there is in legal cases an irrebuttable presumption that people who are about to engage in an activity will comply with applicable law.
For the purpose of the criminal law, what matters is whether Lindon believed that his actions were reasonable, i.e., a subjective test.
Since it was, the court was "on the facts found proved entitled to find that the Respondent had a lawful excuse for the purposes of section 5(2)(b) of the Criminal Damage Act 1971" and "on the facts found proved entitled to acquit [Lindon]" and "criminal proceedings were inappropriate.
At worst a civil wrong had been committed, either nuisance by the appellant [Chamberlain] or trespass by the respondent [Lindon].
"[2] Thus a lawful excuse may be acknowledged by a court to arise when a person honestly but mistakenly believes that the actions are necessary and reasonable.
Depending on the jurisdiction in which the act took place, Jennifer may be allowed to raise the defense of mistake of law in such a scenario.