I'm asked all the time at security "is that water bottle empty?", and as long as I say yes (or chug the remaining contents ), there's no problem. I've never had a TSA agent even hint that an empty bottle is an issue in any way.

The UK does tend to have more stringent, particular rules general, though. It woudln't surprise me if they still have some sort of special rule about empty containers. (It also wouldn't surprise me if the info from last November is way out of date by now.)