You'll find it very hard to get prison access, especially whilst being under 18. The MOJ doesn't let people in for work experience for a number of safety and security reasons, you generally have to be either working or volunteering there regularly in order to justify them admitting you in the first place. The Enhanced DBS costs time and money, I know people who have waited up to a year for theirs to be cleared, which is obviously best spent on more crucial things rather than students wanting experience. They don't even give access to those studying an MSc course and have an interest in using the prison population for their dissertation, it's only if you already have prison access that they'll consider you.