Check your local film office. Also, before you film, make sure you have the proper permits and clearances, or some passersby may think there's something going on and call the police, and you could be put into a potentially dangerous situation.

Does it matter where in the UK?

** If its worth making, it's worth making properly. damn it!**

Honestly, just ask people man.

I'd think it's Kind of odd to first ask an impartial forum for this kind of question. Just ask people you know, for starters.

"Hey parent/friend/colleague/coworker, do you know of anyplace like _____ that's nearby?"

Maybe I'm not understanding your question, but are you really asking where you can find a lake, or forest to film at? Or a nebulous industrial area, or any old scrap yard. If you really have no idea (perhaps because you live in a cave? ?:p? Haha, sorry for being so hard on you buddy), where any such locations are, go for a drive. With no destination in mind, just look for places. This seems like a really big no brainer to me, hope this simply wasn't articulated well enough in your question.

Now, the more specific locations like a quarry, old forts or an abandoned railway station (something especially specific) are a more necessary area for throughout searching. I'd suggest bringing it up with friends again, seeing what leads you can get, then go to the Google machine. Something as simple as "old fort near _____" could potential garner the perfect result. Another place I'd suggest looking is tour brochures. If you're a resident of a city/town/province/country it's easy to forget or be oblivious to more scenic elements of your area of residence. Tourism info could be a good resource.

And hey, all of my "are you for real" comments are in good humour. 🙂