During the flight, a strange object was visible outside of the window, which stayed in the same position for 5 minutes, and flew away

On a flight from São Paulo to London, flying at 36,000 feet above the English Channel at 2:15PM UTC I spotted a strange object flying at a similar altitude from my window seat. I thought it was some kind of military aircraft to begin with, however it quickly changed into a flat, oval type shape, rotated vertically and horizontally. Once this had happened, I began to film the video that I’m going to attach to this report. The still photos were taken when I believed it to be a military aircraft. The object appeared to remain in the same place for a short while, before flying off leaving a white trail behind it. The colour of the object was a grey / black, it did not change, however its shape did, as I mentioned previously. There was only one and I did not see any other strange movements / activity.