At common law a person may use reasonable force in self-defenceR v Rose (1884)15 Cox CC 540) and, in extreme circumstances, may be justified in killing an attacker (R v Clegg  1 AC 482 (HL). Reasonable force may be used in defending one's property (Revill v Newbery  1 All ER 291), and if someone intrudes on one's property at night, one might be justified in treating this as a threat not merely to property, but to personal safety. An occupier of premises (even if he is not the owner) and possibly even a licensee (such as a lodger) may use reasonable force against a trespasser. The Criminal Law Act 1967 permits the use of reasonable force in order to prevent crime, to lawfully arrest a criminal or suspected criminal (or to help in arresting him), or to capture someone who has escaped from lawful detention. The Act extends to both police and private citizens. It is not altogether certain whether the statutory right includes the right to kill.
It is a statutory offence to set spring guns or mantraps, except in a private house between sunset and sunrise. One may use a dog in self-defence if this use is reasonable (see guard dog).
If a person mistakenly thinks that he is entitled to use reasonable force when he is not he may use such force as is reasonable in the circumstances as he believed them to be (R v Shaw  1 WLR 1519). It seems, however, that this does not apply if he made a mistake of law, rather than fact or if the defendant's mistake was based on intoxication (R v O'Grady  QB 995). See also forcible entry.
From: reasonable force in A Dictionary of Law »