Your most sensible option would be to approach the Home Office directly, or HM Prisons, and both of these can be found on the web. However, unless you had written confirmation from a school or tutor that this would be part of an official project, it is extremely unlikely that you would be given such access. They would need considerably persuasion that this was a formal study and not something to satisfy personal curiousity, and even then, your chances are not particularly high. Perhaps you should start by looking through existing interviews - or wading through the book In Cold Blood by Trueman Capote, which analyses the killers of a family in rural Kansas. It goes through the effects caused by environment in childhood, how a person of any of locale can be a victim of hostility, and the influence bringing contrasting personalities together. But be warned, critically acclaimed though it is, it is not pleasant reading.