The law of self-defence is clear and based on common experience. In 1971 the Privy Council in the leading case of Palmer v R  AC 814 held that
It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary.
Section 3 of the Criminal Law Act 1967 provides that
A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large
Section 76 of the Criminal Justice & Immigration Act 2008 adopted the common law principles from Palmer and other leading decisions, without changing the substance of the defence. Section 148 of the Legal Aid Sentencing & Punishment of Offenders Act 2012 amended Section 76, to give further ‘clarification’.
The law permits the citizen to use reasonable force for the following purposes:
- Defence of another person
- Protection of property
- Making a lawful arrest
- Preventing crime
The citizen includes the police officer, to whom the law grants no additional licence to use violence.
What is reasonable is measured by the belief of the person using force. There may be no time for anything other than an instinctive reaction. The law does not require a fine calculation of just what amount of force is needed, but it does require that the use of force should be necessary in the circumstances, as the person concerned saw them. There is no duty to run away instead of using force. “If … in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought necessary, that would be the most potent evidence that only reasonable defensive action had been taken” (Palmer).
There are limits. An obviously disproportionate response would not be reasonable – such a shooting an unarmed burglar in the back, and killing him, as he ran away. This is what occurred in the notorious case of Martin  1 Cr App R 27. The use of violence has to be controlled or else the law will yield to lawless vigilantism.
The Crown Prosecution Service has published Guidance on how to deal with cases of self-defence. Where a person has used force against some one committing a crime, the CPS advises
Common examples are burglary or theft from motor vehicles. In such cases, prosecutors should ensure that all the surrounding circumstances are taken into consideration in determining whether a prosecution is in the public interest.
- Prosecutors should have particular regard to:nature of the offence being committed by the victim;
- degree of excessiveness of the force used by the accused;
- extent of the injuries, and the loss or damage, sustained by either or both parties to the incident;
- whether the accused was making an honest albeit over zealous attempt to uphold the law rather than taking the law into his/her own hands for the purposes of revenge or retribution.
People who think that the law needs changing to free householders to take gross and disproportionately violent action against burglars should read the CPS Guidance. They should also apply some commonsense and ask whether it’s more likely that a burglar will take a weapon if he thinks he’s going to be attacked. Do we want more dead burglars and more dead householders?