The abandoned Harperbury Hospital in Radlett (the UK) used to be a mental health and learning disabilities hospital built on the remains of World War 1 hangers.
It was first opened in 1928 and because derelict in the early 2000s. I visited this site back in 2017 and it was one of the best explorations to date purely because of its size.
The amount of buildings dotted around this site, all abandoned, is incredible and there is no way I managed to explore all of it.
Access was very easy, there were a few patrol guards but nothing the long grass can’t hide you from!