Coroners and Justice Act 2009 s.54
(1) Where are person ("D") kills or is a party to the killing of another (V) D is not to be convicted of murder if -
(a) D's acts and ommissions in doing or being a party to the killing resulted from D's loss of self-control
(b) the loss of self-control had a qualifing triger, and
(c) a person of D's sex and age, with a normal degree of tolerance and self-restraint and in the circumstances of D, might have reacted in the same or in a similar way to D.
(2) For the purpose of subsection (1)(a), it does not matter whether or not the loss of control was sudden.
(3)In subsection (1)(c) the reference to “the circumstances of D” is a reference to all of D's circumstances other than those whose only relevance to D's conduct is that they bear on D's general capacity for tolerance or self-restraint.
(4)Subsection (1) does not apply if, in doing or being a party to the killing, D acted in a considered desire for revenge.
Ahluwalia (1992) 常年被丈夫虐待，之后杀了丈夫。a woman subject to cumulative domestic abuse might lose her self-control after a period of ‘slow burn’, simmering anger which erupts long after the last provocative event. So long as the killing was by way of loss of self-control, and not revenge or premeditation, the defence remained available.
:checkered_flag: diminished responsibility
qualifying trigger
CJA 2009 s. 55(3) This subsection applies if D's loss of self-control was attributable to D's fear of serious violence from V against D or another identified person.
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CJA 2009 s 55(4) This subsection applies if D's loss of self-control was attributable to a thing or things done or said (or both) which—
(a) constituted circumstances of an extremely grave character, and
(b) caused D to have a justifiable sense of being seriously wronged.
subsession (a) is designed to filter out trivial triggers such as 'nagging', crying babies, and sexual jealousy (also explicitly excluded by s.55(6).
Clinton (2012) 老婆不忠，还骂老公没胆量自杀，不爱孩子。 killing was triggered by jealousy, but it is only one of a number of events which cumulatively constituted circumstances of an extremely grave character.
subsession (b)
R v Bowyer (2013) 认识人D入室偷窃，主人V发现后对其女友出言不逊。D杀死了V。 -》court of Appeal confirmed that the defence was not available since D's status as a burglar blew apart his claim that he was justified in thinking he was being seriously wronged. :checkered_flag: also confirm the position of CJA 2009 s 55(6)
CJA 2009 s 55(6) In determining whether a loss of self-control had a qualifying trigger—
(a) D's fear of serious violence is to be disregarded to the extent that it was caused by a thing which D incited to be done or said for the purpose of providing an excuse to use violence;
(b) a sense of being seriously wronged by a thing done or said is not justifiable if D incited the thing to be done or said for the purpose of providing an excuse to use violence;
(c) the fact that a thing done or said constituted sexual infidelity is to be disregarded.
If D is at fault in causing V to use violence, D is not disabled from using V's conduct as a qualifying trigger (Johnson 1989)
Objective element
CJA 2009 s. 54（1）(c)
R v Willcocks (2016) Court of Appeal - personality or mental disorder are not allowed to be taken into consideration.
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