One of my fave sites ever is now on serious borrowed time. This was once a huge MOD factory for producing bombs, shells and gas for all branches of the military. It was finally closed and left to rot in the 1980s when the industry was farmed out to the private sector.
One half has been demolished and the land used to build a new "super prison" and the reamining half is showing serious signs of following soon. There's still some bits and bobs around and it's worth a look if you're passing. The fence is now very secure, probably a result of the prison construction going on next door.