This is exactly the kind of thing that keeps me down rabbit holes at 2am. What were the conditions like - clear sky, any wind? And did it make any sound at all? That's usually the first thing I try to work out because drones have a pretty distinct buzz and most natural stuff like ball lightning or plasma phenomena tends to move in ways that feel almost random rather than just sitting there.
The tree line detail is interesting as well because theres a few cases around Scotland where people have reported similar things near woodland edges specifically. Not sure if thats significant or just coincidence.
Did you try filming it on your mobile or did it happen too fast? Even a shaky clip would be something to go on. Would love to hear more details on the shape and how long it was actually hovering for.