It is acceptable for you to defend yourself in a reasonable manner and force if you are placed in a situation where someone is assaulting you and prevent them from causing you any more harm or injury.

Although an act of defence, you cannot simply pull out a knife that you happened to have with you or any weapon that you decide to use to defend yourself because this would easily be identified as an offensive weapon. This can be considered a disproportionate response to the attack during that instance. If the attack was to continue, and you feared for more violence or for your life, you can then use whatever it is available at the time, for example, the knife you have with you.

The common law defence of self-defence applies where the defendant uses necessary, reasonable, and proportionate force to defend themselves from imminent attack.

It is considered to be a complete defence to all non-sexual offences involving the unlawful use of force, specifically anything from battery to murder.

As the defence results in a complete dismissal, the courts have interpreted the defence in a restrictive way so as to avoid dismissing too easily. For example, courts will not usually acquit the defendant just because of the notion that the force used was reasonable.

The jury will assess objectively whether or not the force used was reasonable—and not simply based on what the defendant thought at the time. Self defence in the UK can include:

• defending yourself;
• defence of another;
• prevention of crime; and
• lawful arrest and apprehension of offenders.

Self defence and prevention of crime derive from different sources:

• Common Law — defence of the person;
• Criminal Damage Act 1971 — defence of property; and
• Criminal Law Act 1967 — arrest and prevention of crime.

In England and Wales, anyone can use reasonable force to protect themselves or another individual, to carry out an arrest, or to prevent crime. People are protected from prosecution as long as they act honestly and instinctively in the spur of the moment.

“Fine judgments” over the level of force used are not expected, as per the Crown Prosecution Service. This basically means that someone can claim they attacked in self-defence if they genuinely believed they were in peril—even if in hindsight they were utterly wrong.