As far as I am aware all prisoners serving determinate sentences, regardless of length, are automatically released after serving half. What happens when they are released differs between those sentenced to less than 12 months and those sentenced to 12 months or more.
Those sentenced to short sentences are simply released, not on licence. If they reoffend during the second half of their sentence period they may be required to serve some or all of the remainder. Those sentenced to 12 months or more are released at the halfway stage and are “on licence” for the remainder. The licence has conditions (e.g. don’t go to certain locations, don’t associate with certain people). If they reoffend or break the terms of their licence they may be required to serve some or all of the remainder.
Those sentenced to indeterminate sentences or to life have a minimum tariff set by the judge. They will not be released until that period has elapsed and may serve longer. Their release or further detention is determined by the Parole Board.



