Quicksand Fans

I've seen the quicksand map and a few others now, yet I'm still unable to find any deep, smooth (or reasonably un-public) pits to try out. I've done [what feels like] endless research into the area I live, for about a 30-mile radius. Yet I'm not finding anything :/

I won't mention my location but I will say that I'm looking for sinking spots in England in particular. Also, I don't plan to visit any suggested places until I've done research!

Any good suggested sinking locations are appreciated