That's not a complete picture. The "half term" does not apply
to prisoners serving life, for which judges hand down minimum tariffs
to prisoners considered a danger to the public, e.g. some sex offenders, terrorists, etc
to prisoners who haven't exhibited 'good behaviour' while inside
to prisoners (whether guilty or innocent) who assert their innocence while in jail
Also, prisons are full of individuals with mental health problems. And they're overcrowded. Mike's suggestion that some kinds of offence could be treated effectively outside prison has a lot of merit.