"it is important to ensure that all those acting reasonably and in good faith to defend themselves, their family, their property or in the prevention of crime or the apprehension of offenders are not prosecuted for such action"
"Prosecutors should exercise particular care when assessing the reasonableness of the force used in those cases in which the alleged victim was, or believed by the accused to have been, at the material time, engaged in committing a crime. A witness to violent crime with a continuing threat of violence may well be justified in using extreme force to remove a threat of further violence.
In assessing whether it was necessary to use force, prosecutors should bear in mind the period of time in which the person had to decide whether to act against another who he/she thought to be committing an offence."
http://www.cps.gov.uk/legal/section5/chapter_d.html