This explanation provided by Terry Platt: " I think that it has to be an aircraft, as the lower 'flash' is well off the main trail. I don't think anything in orbit (apart from the ISS) would subtend a sufficiently large size to account for the displacement. Some aircraft strobes
are xenon flash tubes and give millisecond pulses, so I think that you got one
wing tip at the top and the other wing tip at the bottom. The main trail is
probably cabin lights or a constant fuselage light".