UK Vigilantism and the Law which allows it
As a vigilante you will invariably come across a time where you will have to explain yourself legally. There is no specific law which explicitly permits or criminalises vigilantism, but there are cases and laws which allow for it to be undertaken while staying within the law.
Self-defence in the UK is legal and is a full defence - more than that. It is a justification rather than an excuse, essentially saying that you have not committed a crime. It has been in common law for centuries and is enshrined in the law in various places. Self-defence here also means the prevention of a crime.
You must answer two questions to be able to use self-defence:
Was the force necessary?
Was the force used reasonable in the circumstances?
A person may use such force as is [objectively] reasonable in the circumstances as he [subjectively] believes them to be - R v Owino
You do not have to wait to be attacked or for another person to be attacked: A man about to be attacked does not have to wait for his assailant to strike the first blow or fire the first shot; circumstances may justify a pre-emptive strike - R v Beckford
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This is not just created through case law, but through the codified law itself:
Criminal Law Act 1967:
Use of force in making arrest, etc.
(1)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.
Reasonable force means that you cannot beat an attack to death unless your life is at stake - had you not taken that action, you would have died. It does mean that force, such as tackling or striking, is allowed under the law, if used to prevent a crime or stop a perpetrator’s escape.
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You can arrest someone committing a crime under the Police and Criminal Evidence Act 1984:
Arrest without warrant: other persons
(1)A person other than a constable may arrest without a warrant—
(a)anyone who is in the act of committing an indictable offence;
(b)anyone whom he has reasonable grounds for suspecting to be committing an indictable offence.
(2)Where an indictable offence has been committed, a person other than a constable may arrest without a warrant—
(a)anyone who is guilty of the offence;
(b)anyone whom he has reasonable grounds for suspecting to be guilty of it.
(3)But the power of summary arrest conferred by subsection (1) or (2) is exercisable only if—
(a)the person making the arrest has reasonable grounds for believing that for any of the reasons mentioned in subsection (4) it is necessary to arrest the person in question; and
(b)it appears to the person making the arrest that it is not reasonably practicable for a constable to make it instead.
(4)The reasons are to prevent the person in question—
(a)causing physical injury to himself or any other person;
(b)suffering physical injury;
(c)causing loss of or damage to property; or
(d)making off before a constable can assume responsibility for him.
Essentially, if somebody is committing an indictable offence (murder, manslaughter, robbery, rape, supplying drugs and many more), and you know they are doing the offence or have reasonable grounds to think that they are, you can arrest that person without a warrant if it is clear that it is not practical for the police to do it instead. It is advised that once you make the citizen’s arrest that you immediately call the police so an official arrest can go ahead.
