Read the link for the answer,AOG..
A judge always has the power to direct an acquittal . Have you never heard of 'no case to answer' ? That's where the judge directs the jury to acquit. In this case, having heard legal argument and expert evidence from psychiatrists, the judge ruled that the defence of 'diminished responsibility' which was being raised was unanswerable. The prosecution, led by Queen's Counsel, agreed.
'Diminished responsibility' is a defence peculiar to murder cases. It was created by section 2 of the Homicide Act 1957. It is not available in any other case.If it succeeds, the jury return a verdict of not guilty to murder but guilty of manslaughter.
It applies when the accused was suffering from such 'abnormality of mind' at the time of the offence as 'substantially impaired their mental responsibility for [their] acts or omissions in doing or being a party to the killing'
It is raised in all manner of murder cases, not just ones of clear psychiatric condition. It's quite common in 'battered wife' cases where the woman has been subjected to persistent abuse and suddenly 'snaps' and picks up a weapon and kills her abusive husband. It is sometimes, somewhat misleadingly, known as the defence of provocation' because, in extreme cases, someone might be so provoked that they kill the provoker , in what might be termed 'a moment of temporary insanity'