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)

(https://www.merriam-webster.com/dictionary/illegal)

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.