I'm happy to be corrected on this, but it seems to me that there is no general lawful excuse
defence in UK law. There are mitigating circumstances
, such as "I'm guilty but prevented a greater harm", but that's a different concept, and not a defence.
UK legislation often carries exemptions, eg speed limits don't apply to an ambulance if they would hinder its use. (RTRA 1984 s87
Sometimes the phrase is used explicitly, eg:
... Criminal Damage Act 1971
: "A person who without lawful excuse destroys or damages any property...", and "lawful excuse" has its own description. The similar Irish law was famously used regarding breaking an aircraft with an axe
... Armd Forces Act 2006 s1
: a soldier mustn't "without lawful excuse" give an enemy useful information.
Searching for "lawful defence" in Bailii
finds many results about criminal damage, but none in the past 100 years as a defence to other charges.