The head banging isn't some neurological disorder, jesus.
Anyone who works in a UK custody suite will tell you tons of prisoners want to knock themselves out, seem mentally unwell to aid their case, or are in a withdrawal rage, or just want to try and kill themselves somehow will all do exactly the same thing. It is the only way they can injure themselves in a cell!
They will either tie their top around their neck (if not in anti strangulation clothing) or smash their head against the toilet bowl.
Him doing it in custody is pretty irrelevant and genuinely nothing significant at all.
MOO.