The prison service is not allowed to tell anyone the location of any particular prisoner, Yes it's part of the human rights act.
You can send a letter to the Prison Service with the persons full name on it and they will pass it on. Best if you can put the date of conviction and the court as well so they can be sure of sending it to the correct person. With over 86,000 in prison now there are a lot of similar names.
Be warned, the prisoner does not have to reply or even accept the letter, some of them want to remain anonymous.