Yes, Teribus, at that time, one served two thirds of any sentence over 4 yrs. And half of any sentence less than 4 yrs. But all that depended on good behaviour inside, few 'Adjudications' (brought before the Governor for misdeeds) and being subject to the judgement of the Parole Board. Time already served On Remand was deducted too. I don't know if things are the same now. A sentence of 6 years was fairly common for offences such as I described above. But never such a short term for murder.
I have visited in Parkhurst Prison several times, and there were men there serving 20yrs for murder. Near the end of their term, they would be transferred to a C Cat prison or even a D Cat (open prison) for the last year or so.
I'm just amazed (and rather disgusted to be honest) that the judge seemed to be doing her utmost to give the chap the easiest possible sentence. Why?