He can not be held legally responsible with a conviction because he doesn't have the mental capacity to aid in his own defense.
with his low IQ a good dose of adolescent testosterone and he becomes a walking hormone with no internal controls at all.
This is a very sad situation and the only real answer is a life of institutionalization. I do not know if the British system, or any system is set up to care properly for this type of person.
Once he becomes a serious danger to others ,or more likely seriously harmed or killed ,there are special prisons for the mentally unstable.
Broadmore,Ashworth,Rampton.