I think mental/disorder/illness/psychiatric are used interchangeably in the UK when discussing policy.

It is widely accepted that locking up people with mental disorders of any sort is futile (Bradley Report http://www.mac-uk.org/wped/wp-conten...ec-Summary.pdf), but to treat them would cost money (and is not exactly a vote-winner compared to schools and hospitals): mental health in prisons

However, even the current government is supportive: https://www.gov.uk/government/speech...custody-speech