Jake all prisoners go to a closed prison first , called a distribution prison, while they are there the prison service decide on the category they will be A,B,C or D and which jail will be the 'permanent' one. Huhne will be a cat D so eligible for open prison.
Cat D means low escape risk and a short sentence for a non violent crime. (it can also be a long term prisoner near the end of sentence) .
em10
All prisoners sentenced to 2 years or less are automatically released at the half way point . 2 to 4 years get automatic release at the 3/4 point , over 4 years means no automatic release , they have to apply for parole/ review and the judge will set a date that is earliest possible review date. This is where a lot of people get worked up at ' Short sentences' the prisoner will very rarely actually get a review at that date, it could be years later.
Very rarely does 'Life' actually mean life in jail only the very worst offenders (Ian Brady etc) get a 'Whole Life Tariff'