Intriguing.
Being in London you'd expect this to be captured by more than one person using camera phones etc., the event lasts for a good length of time so it's reasonable for people to start filming in time.
It'd be interesting to see if/how it differs on other footage as I think that's the real acid test. As long as it's just one video, it could have been faked or it could be a light problem or a defect with the camera. Other people looking up doesn't say much as they could be looking at anything, like a helicopter, and the footage could be manipulated from there.
I think it's a very good piece of footage and although I haven't got much in the way of knowledge with special effects it'd be good to see how it stands up to scrutiny to somebody who does. I'm not a great believer in 'aliens coming to Earth' but it's far from impossible. The movement of the big object and the way it seems to duck behind the clouds is the bit that gets me the most because up to that point I was thinking it might be a balloon.
