Moved very short sentence prisoners come out automatically at the half-way stage - Automatic Date of Release - ADR. Over 12 month sentences, a risk assessment is done by prisons to determine whether prisoners can be released for home leave, hostel leave or on an electronic tag when they reach the relevant stage of their sentence, depending on whether they can be released safely. Violent and serious offenders are not allowed this privilege early in their sentence, only when they move to open conditions and have demonstrated progress in all the risk areas, such as substance abuse, attitude to offending, taking responsibility, behaviour in prison, problem-solving abilities and another 6 or 7 areas (Offender Assessment System - OASys - can be found on Google). If they abuse this privilege, they remain inside until their Latest Date of Release - LDR. That's the date beyond which it would be illegal to keep them in prison.
For those with long sentences, they have to work towards a Discretionary Date of Release - DCR. Many go past this date because of their attitude and behaviour. I don't know the statistics, but one group can't be compared with another.
Prison Governors (Grade 1) are personally responsible if they release a prisoner and they commit a further imprisonable offence.