Re: Automatic halving of sentence
It's because there is no facility to extend a sentence unless another crime has been committed or new evidence received and it is returned to court.
Therefore you can't hand down a 9 month sentence but if they a very badly behaving prisoner extend it to 18 months. SO the court first doubles it to 18 months and then you can halve it for good behaviour or other circumstances. There are a few sentences which automatically qualify for a half life and a few that qualify for full term.
However it is in the prison service's (government's) interest to have well behaving prisoners who are let out of their very expensive prison cells and back into the wider world with as little hassle as possible. The prison system is much cheaper to run that way.