My understanding is that two experts need to agree on the diagnosis of a recognised psychiatric condition. The burden of proof is very, very high for this kind of defence - this is from Wikipedia on the section of English law but a good summary IMHO:
Section 2 of the Homicide Act 1957 states: (1) Where a person kills or is party to a killing of another, he shall not be convicted of murder if he was suffering from an abnormality of mental functioning which -
(a) arose from a medical condition
(b) substantially impaired D's ability to do one or more of the things mentioned in subsection (1A), and
(c) provides an explanation for D's acts and omissions in doing or being a party to the killing.
(1A) Those things are -
(a) to understand the nature of D's conduct;
(b) to form a rational judgment;
(c) to exercise self-control.
Based on this, MOO, he'll never get such a defence to stick, and I doubt his lawyer(s) will even let him try.