The site has been very secure for years, with 8-10ft palisade fencing and 24hr security. I wouldn't say the Dartford youth are known for behaving.
The site is now being developed into residential homes. The older buildings are staying, but it looks as though the later are being demolished and hundreds of new builds squeezed in, as per usual. I doubt that much of the greenery will survive.
It's surprising where you can go if you look like you belong there. In an earlier incarnation, as an engineer, I have wandered around working factories, unaccompanied, here and on the continent, similarly dressed and never been challenged by anyone. (I must dust off the hard hat, ear defenders and hi viz jacket and stick them in the car in case an opportunity arises )