Just to be a bit pedantic (to which I�m prone from time to time) on a point raised by anotheoldgit, they will not get time off �for good behaviour�.
This principle was abolished some time ago. Prisoners used to be able to earn up to one third off their sentences for good behaviour, and it was in the gift of the prison governor to reduce that remission if bad behaviour occurred. (He would award, say, fourteen days loss of remission).
Some time ago (I cannot recall exactly when, but around the time that �Human Rights� in prisons came to the fore) this principle was abolished. Using the usual perverse logic that seems to pervade such matters it was ruled that the remission was a right that could not be taken away save by a properly convened tribunal, and the prison governor did not constitute such a tribunal. Effectively it was said that in acting in such a way governors were imposing additional prison sentences, which they were not entitled to do.
So now prisoners can misbehave as much as they like and receive no penalty unless full and formal criminal action is launched. This is rarely done save in exceptional circumstances as their misbehaviour often does not cross the criminal threshold.
Now all prisoners with defined sentences serve a maximum of half of their sentence inside (regardless of their behaviour) and are released to serve the remainder �in the community�. In practice this second half usually consists of one or perhaps two interviews with a probation officer within the first few weeks of release. Whilst in theory they can be called upon to serve the remainder of their sentence in custody should they reoffend, few are.