Possibility of early relase for a SO?

Alright so i'll get down to the nitty gritty here. What are the chances of a SO (sex offender) being able to get out of prison early on tag or something like that? Lets say that this SO is he/she is in prison with this being their first offence, they have been sentenced with two and a half years and will have to register as a SO for the rest of their lives. Would their be anyway of them being able to get an early release? From what i'm aware prisoners only fulfil half of their sentence for good behaviour but is there any other way someone could get out earlier than that?