Completely anecdotal, but growing up in the early 90's a friend's family kept sheep, and knew another local farmer who had found a sheep carcass up a tree on his property. There are no native UK species that ought to be able to pull that off...
The range data does freeze from some point near the end of the video, so does not reflect the actual range estimate during that segment. But otherwise the range is in the thousands of meters, which would rule out a small object such as an insect...
This is what frame 984 of Gimbal looks like, showing the object's geometric properties detected by the method described here.
Clearly the accuracy of the method is heavily degraded on such frames where the object makes sudden movements leading...