Prisoners Rehabilitation Or Punishment? - By Mum Of Prisoner

My son has been in prison for 4 years now and has 6 left to do, or thereabouts. If you ask me whether he deserves to be there, my answer is, yes. But I am baffled with what the UK system want from him. Is he in prison for punishment or rehabilitation? I was led to believe that was the sole purpose of prison, or am I wrong altogether? My son has never been able to 'settle' in prison because once he adapts to one, he is moved to another, which no doubt sets him back. Is rehabilitation what the UK call locked up for 20 + hours a day in a cell watching TV? Or is that punishment? I watched, Question Time, the other night, and if that is what type of people are running the system, then we'd better give up now!