I was in a local park in addiscombe, croydon, uk with my dog and a group of approx eight other dog walkers. i saw the black donut shaped object which appeared to be approximately 18" across enter the park airspace from the east above houses approximately 100 feet away. its height was only about 30 feet above the ground, so only just above the roof tops. it was traveling west very slowly, much slower than walking pace. it slowly ascending in height and at some points it stopped moving and was completely static. it then continued to rise in height. i watched it do a 180 degree flip so the bottom of the object became the top this happened at least twice. when it flipped it appeared to be in complete control of the movement not bouncing around like it would if the wind was affecting its movement. i pointed the object out to the other dog walkers and about five of us watched it continuously all the time it was gaining height. at one point a passenger aircraft also crossed the park. the passenger aircraft had either taken off or was getting ready to land, the aircraft seemed to be much lower than we normally observe maybe 2000 feet at the point we saw the aircraft the donut object was above it at least twice as high as the aircraft. it was i thought unusual that we could still clearly see the object at such a height when it was relitivly so small in comparison to the aircraft although i couldn't see the central hole of the object. the object continued to ascend until it was too small to see any more.