English law provides for the right of people to act in a manner that would be otherwise unlawful in order to preserve the physical integrity of themselves or others or to prevent any crime. It is provided in both common law and more specifically in the Criminal Law Act (1967). If such a defence is proved to the satisfaction of the court then the person is fully acquitted of the charges against them.
The act of protection must fulfill a number of conditions in order to be lawful. The defendant must believe, rightly or wrongly, that the attack is imminent. While a pre-emptive blow is lawful the time factor is also important, if there is an opportunity to retreat or to obtain protection from the police the defendant should do so - demonstrating an intention to avoid violence. However the defendant is not obliged to leave a particular location even if forewarned of the arrival of an assailant.
The other key factor is reasonableness - the defendants response must be necessary and in proportion to the nature of the attack. The harm inflicted on the assailant must not exceed the harm being avoided by the defendant. However like imminency the nature of the defence rests on the defendant's belief, whether their actions were in proportion to the circumstances they believed existed.
It has been argued, with some force, that the above qualifications contain a gender-bias. If the attacked party is considerably weaker than the assailant then to offer an immediate response would be effective in only encouraging greater harm to be inflicted upon the defendant. While if the defendant waits and strikes back later then self defence cannot be applied under current law. Certain groups propose a self-preservation defence.