Been there mate. That feeling where you just can't shift it from your head, keeps replaying. Thats usually the sign it was something worth paying attention to.
What part of the country are you in? That matters more than people think. Certain areas have consistent activity over long periods and it's not random.
Did it make any sound or was it completely silent? And roughly how high up would you say it was - treetop level or higher? The behaviour of it interests me more than the shape tbh, the way these things move (or don't move) tells you a lot more than trying to slap a label on what it looked like.
I've seen two things over the Norfolk fens I still can't explain and I've been doing this a long time. Post as much detail as you can remember while its still fresh. Don't let people talk you out of what you saw.