Right so I've had a similar thing happen to me out in Suffolk, couple of years back now. Silent, slow moving, low over the treeline - just sat there for a good few minutes before drifting off to the east. No sound, no flashing nav lights, nothing that made any sense.
The bit that gets under your skin is the silence isn't it. Helicopters, drones, planes - they all make noise. Whatever I saw made absolutely none.
What sort of shape was it? That's usually the first thing I try to nail down when I'm recounting these things. Round, elongated, irregular? And how long did you watch it before it moved or disappeared? Details matter more than people realise, especially if you want anyone to take you seriously when you report it.
Don't let people talk you out of what you saw. You saw what you saw. Post back with more details when you've had a chance to gather your thoughts, there's a few of us on here who'll actually engage with it properly rather than just shouting weather balloon at you.