My understanding is that the original legal intent of psychiatric defenses was to provide a recognition of medically documentable physiologic deformities such as a brain tumor or profound neurological birth defect that caused the brain to function incorrectly.....they weren't intended for jihadists who drink and smoke pot.
Yes, mental illness has to be proven first. I hope the Brits are bright enough not to fall for demands that he be pstchologically tested by a Muslim psychiatrist. A competent evaluation, given by a court appointed doctorate of psychiatry is enough.
Speaking of Muslims and mental illness, I've had Muslim clerics insist that Muslims don't suffer from mental illness, Islam is supposed to protect them from it. The verbal gymnastics you get when you ask why there are Muslim psychiatrists are interesting.