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Under UK law, trespassing is a civil not criminal matter and so by some definition it is not illegal.

Still illegal, just not a crime.

No, it's unlawful, not illegal.

Definition of illegal: not according to or authorized by law : unlawful, illicit; also : not sanctioned by official rules (as of a game)

In legal terms, illegal and unlawful are not synonymous. In the UK, trespass is only illegal in certain circumstances: https://cps.gov.uk/legal-guidance/trespass-and-nuisance-land

Beyond that, it's only unlawful.