Self-defence is a complete defence and results in an acquittal. Defence of the person is governed by the common law, with the basic principles set out in Palmer v R, [1971] AC 814; approved in R v McInnes, 55 Cr App R 551:

“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.”

Arrest and the prevention of crime are governed by the Criminal Law Act 1967, which specifies a similar test:

“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.”

A person may use such force as is reasonable in the circumstances for the purposes of:

• self-defence;
• defence of another;
• defence of property;
• prevention of crime or
• lawful arrest.

In assessing the reasonableness of the force used, two questions are asked:

1. Was the use of force necessary in the circumstances, i.e. Was there a need for any force at all?
2. Was the force used reasonable in the circumstances?

These are to be answered on the basis of the facts as the defendant honestly believed them to be (R v Williams (G) 78 Cr App R 276; R. v Oatbridge, 94 Cr App R 367). The Criminal Justice and Immigration Act 2008 provides that, if it is established that the defendant did genuinely hold the belief the force was necessary, he may rely on that belief to establish the force used was reasonable whether or not it was a mistaken belief.

However, the second question has an objective element: given the circumstances as the defendant believed them to be, would a reasonable person regard the force used to be reasonable.

1. Mental illness is relevant to the first question: Oye [2013] EWCA Crim 1725

Mental illness as a contributing factor, including as the cause of a mistaken belief, is applicable to the question of whether the force was necessary (Oye [2013] EWCA Crim 1725). However, it has been found delusions were not relevant to the second limb, the reason being that “An insane person cannot set the standards of reasonableness as to the degree of force used by reference to his own insanity.” However, the conviction was overturned in favour of a finding of insanity.

Both Canns [2005] EWCA Crim 2264 and Oye [2013] EWCA Crim 1725 could not conceive of any circumstances that would provide an exception to this.

R v Martin (Anthony Edward) [2001] EWCA Crim 2245 involved the conviction of murder for the killing of an intruder into Martin’s home. One of the grounds of appeal was that of self-defence, citing his paranoid personality disorder. The court found that “…we would not agree that it is appropriate, except in exceptional circumstances which would make the evidence especially probative, in deciding whether excessive force has been used to take into account whether the defendant is suffering from some psychiatric condition.” However, his conviction of murder was overturned and he was convicted of manslaughter via diminished responsibility.

2. A defendant is not entitled to rely upon a mistake of fact which is attributable to voluntary intoxication: R v O’Grady [1987] 85 Cr App R 315; R v Hatton [2006] 1 Cr App R 16

R v Taj [2018] EWCA Crim 1743 expanded the definition of “attributable to intoxication” to encompass the “immediate and proximate consequences” of substance misuse, including the induction of poor mental health. Though it was noted in the judgement that this conclusion “does not extend to long term mental illness precipitated (perhaps over a considerable period) by alcohol or drug misuse”.