Right so this is exactly the kind of thing that sticks with you isn't it. That feeling the morning after where you're just going about your day but your brain keeps pulling you back to it.
Few questions - how long did it hover for roughly? And was it completely silent or was there any sound at all? That's usually the detail that helps narrow things down because drones have a pretty distinct buzz even at distance, and most conventional aircraft don't just sit there.
I've done loads of night vigils out in rural Essex and the tree line sightings are honestly some of the most unsettling because theres no real reference point for size or distance. Something can look massive or tiny and you genuinely can't tell.
Post any photos or video if you managed to grab your phone in time, even blurry footage tells you something. And write down everything you remember right now before the details start fading, trust me on that one.