Here's the link that was shared with me on Twitter: UFOs seen in sky over London
Something about it strikes me as fake but I'll let the experts judge.
Can anyone tell me how can this happen if it's not a genuine UFO sighting?
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It is unlikely that real world objects would happen to speed-up/slow-down exactly in time with the camera movement.
This is highly suggestive that the UFOs are fakes, added in post-production, with a small glitch in their camera-tracking.
There have been some claims that the work was done by Beam.TV, who have an office nearby the video you cited, but I haven't found any substantive evidence of that.