The question of "good behaviour" is tricky.
Prior to this half-sentence nonsense (about 1995, I think) prisoners were eligible for release when they had served two thirds of their sentence "subject to good behaviour". The judgement of good behaviour was with the prison governor who could order "loss of remission" for bad behaviour. This was ruled unlawful as it was (perversely in my view) ruled that the loss of remission was an additional sentence. The prison governor was (rightly in my view)not deemed to be a "properly convened tribunal" and could not order the (perversely termed) "additional sentence.
If "good behaviour" is to be a requirement for early release a system of tribunal will have to be devised to consider those cases where the standards for good behaviour were allegedly not met. And we all know what a pig's ear the government, aided and abetted by M'Learned Friends, will make of that.