I’m going to try to answer his one. I hope a criminal solicitor chips in now.
The test for the diminished responsibility partial defence to murder is in s2 Homicide Act 1957. It looks from the wording that the defence could potentially apply to a premeditated killing.
The guidelines on diminished responsibility in Sentencing Council, ‘Manslaughter: Definitive guideline’ (published 31/7/18, enforcement date 1/11/18) list “A significant degree of planning or premeditation” as a non-statutory aggravating factor increasing seriousness, and “Lack of premeditation” as a factor reducing seriousness or reflecting personal mitigation.