Pete's bang on the money here. It's not about finding the site, it's about getting permission to film there.
There are usually a number of very good reasons why they put bloody great fences around places like that.
As an alternative, try booking a rehearsal at Panic Button Studios on Platt's Eyot in the Thames at Sunbury/Hampton.
Maybe not quite as 'abandoned factory' as you want, but certainly a huge slice of 'industry that time has left behind' with an old WW2 airfield sort of feel to it, verging on an episode of The Prisoner.