Interesting one this. What colour was the light and was it making any sound at all? That's usually my first two questions because the answers tend to narrow things down pretty quickly.
Seen similar over Alton Water reservoir near me a few years back, orangey glow, completely silent, moved dead slow then just stopped. Never got a satisfying explanation for it. Not saying it was anything exotic but it definitely wasn't a drone or a lantern, I know what those look like.
If you've got a decent camera get out there tonight and set up if the weather holds. Even a shaky photo is better than nothing. And try to get something in the foreground for scale, trees or a rooftop, whatever you can manage. Makes a big difference when people start picking the image apart.