There aren't too many truly abandoned prisons out there in this country - the closest thing to a prison being the one in Hull.
There are a few old Victorian prisons that closed over the last few years but if they are anything like the one in Reading that closed, they are surrounded by a ridiculously high wall and the only feasible 'ways in' as such are a single window and roller shutter built into the wall. Regardless of that the recently closed ones are all still 'not quite derelict', I know they were offering tours around one of them recently.
There is a big one in Lille that loads of people have done but it's a wreck.
A prison is one of the few kinds of buildings I've not done, so I may have to pay a visit to the equally ruined one in New Jersey when I'm next in America.