Section 44 is where you get stopped and searched under "terrorism" laws. It's frequently mis-used and most officers overstep it's boundries, but probably applicable at a power station, disused or otherwise.
It's not pleasant, but if it does happen, keep calm and be reasnable. If they step way over the line, you can take action later. They do NOT have the right to make you delete any images , that can only be done with a court order.
Have a read through this lot, it should tell you a bit more.
http://www.google.co.uk/search?q=section+44+search
This, is something you should print out and keep in your bag. Most useful for showing an overzealous oficer that he's probably about to make a big mistake if he persists in claiming he can do things he's not allowed to.
http://www.sirimo.co.uk/2009/05/14/u...ers-rights-v2/