Asking for permission will fail 95% of the time, Councils and property owners don't really accept that people would like to look around their derelict properties. The request will fall at the first hurdle when someone raises Health and Safety, every now and then you will be lucky enough to stumble upon someone who will be accommodating and let you in with a few caveats.
But in my experience, it is much less hassle to hop the fence and worry about getting caught than asking for permission. And if you ask for permission, have it denied and then go anyway and get caught, I think the outcome is a lot worse (would have to check tbh).
It is undergoing conversion at the moment, which means the contractors will boot you off site in an instant.
on a side not, dereliction + models is hardly original.