Criminal psychology doesn't exist in the UK, it's forensic psychology over here. Criminal psychology is an American term.
You can do a straight psychology degree or one in forensic psychology, as long as it's accredited by the BPS, but many choose a straight psychology degree as it may provide more options later down the line. You'd have to then do a masters degree in forensic psychology regardless of your undergrad so specialising so early isn't generally necessary.
Forensic psychologists work in all different kinds of institutions, not just in prisons and not just with 'convicts' (or offenders, which is the preferred term). You can work in secure hospitals, in the community, within probation or the police, or go into research or academia. Lots of forensic psychologists work in clinical settings and vice versa.