Right so this is exactly the kind of report that gets my attention, because the treeline detail matters more than people realise. A lot of genuine anomalous sightings happen at that specific threshold - the edge between open sky and wooded cover - and theres a theory going back to John Keel's work that whatever these things are, they seem to use that boundary deliberately.
Few things would really help narrow it down. Was it making any sound at all? That's often the first thing that separates a drone or conventional craft from something genuinely unexplained. Also, did it move in a straight line or was the motion more... stuttery, hovering in place then repositioning? And colour of any light it was emitting.
I've had two sightings myself over the Brecon Beacons over the years and both times the thing that stuck with me most was the silence. Just absolute silence where there should have been noise. Genuinely unsettling in a way that's hard to articulate after the fact.
Get as much written down as you can while its fresh. The small details fade faster than you'd expect.