Unlikely to be a raven in this area, and cormorants DO get quite high up. Was it soaring with still wings, or beating them at all? How were the wing beats? Did they look as if they were flicking back from the wrist?
One possibility is white stork. At height, it canlook very black and thus changes shape. I have seen them over Kent. The predominant whiteness can let them "disappear" when at the right angle to the light.