Absolutely right, but I think the difference lies in the outcome options within law. Kidnap someone, even while not of sound mind, can only be a guilty of kidnap outcome. Rape someone, even when not of sound mind, can only be a guilty of rape outcome. To have evidence of psychological considerations would only affect sentencing recommendations, such as in what facility the sentence is served, for those two crimes. Murder is quite different as there is another possible outcome in that yes, you caused someone's death, but due to the evidence of psychological considerations one CAN be found guilty of manslaughter as opposed to murder.
I think the not guilty by reason of insanity defence can be applied for any criminal charge: Criminal Procedure (Insanity) Act 1964
This is what it says in regards to murder specifically and diminished responsibility:
"
Evidence by prosecution of insanity or diminished responsibility
Where on a trial for murder the accused contends—
(a)that at the time of the alleged offence he was insane so as not to be responsible according to law for his actions; or
(b)that at that time he was suffering from such abnormality of mind as is specified in subsection (1) of section 2 of the Homicide Act 1957 (diminished responsibility),
the court shall allow the prosecution to adduce or elicit evidence tending to prove the other of those contentions, and may give directions as to the stage of the proceedings at which the prosecution may adduce such evidence."
But I am not a lawyer.