Interesting post. As loosecanon has already said, it's a civil matter in England/Wales and Northern Ireland (not sure about Scotland?) unless you organise a rave or a mass protest. Old hospitals and mental institutions seem to be popular venues - a touch of the macabre perhaps but aspect that has me puzzled is that many photographers making fine art images of nude models seem to favour abandoned buildings. Maybe it's the potential for isolation and privacy or perhaps it's the generally available space?