It’s not actually, the primary purpose of sending people to prison in England and Wales is punishment. And that's fair enough, but we have to acknowledge that harsher punishments and poor conditions inside prisons encouraging reoffending when people get out.
If we wanted to rehabilitate people we wouldn't lock them up for increasing longer periods of time and expect it not to have a detrimental effect on the ability to live on the outside again.
Whether Colin Pitchfork can be rehabilitated is another issue, someone who admitted to exposing himself to over a thousand women, raped and murdered two fifteen year-old girls and diagnosed as a sexual psychopath seems to be beyond rehabilitation.