immediate protection of someone else's property

"The case involved a group of activists who broke into and vandalised a Brighton factory run by engineering firm EDO MBM. The company was making parts for use in the bomb-aiming equipment on Israeli F16 warplanes. The invasion shut the factory for a week and caused £187,000 worth of damage. But five men and women who appeared in court claimed they had done nothing wrong under criminal damage law. The law says someone is not guilty of causing damage if they believed it was necessary for the immediate protection of someone else's property." (thanks Chris)