well from your link sources, David Clark the UFO guy says(below), so not sure why you are saying no "normal" explanation ever been considered likely.
DAVID CLARKE Sheffield Hallam University, UK e-mail: email@example.com
. PAUL FULLER MARTIN SHOUGH National Aviation Reporting Centre on Anomalous Phenomena (NARCAP) e-mail: firstname.lastname@example.org
Abstract—Unusual atmospheric phenomena (UAPs) were observed in daylight by multiple observers on board two civil aircraft in widely separated locations. We summarise results of an investigation based on radio communications reporting events in real time to Air Traffic Control (ATC), ATC radar and weather radar recordings, Civil Aviation Authority (CAA) documents, witness interviews and statements, and other sources. We describe attempts to explain the phenomena with the help of expert specialist advisers and professional resources in the fields of meteorology, atmospheric optics, oceanography and geophysics. We are able to show that widespread media stories describing enormous phenomena up to a mile wide and detected by radar were based on speculation and misunderstandings. Many news reports were grossly exaggerated and inaccurate. However, we are unable to conclusively identify the UAPs observed. It proves possible to eliminate a number of theories with a fairly high level of confidence, leaving us with two types of phenomena—a rare atmospheric-optical effect or an earthquake precursor—both of which have the potential to explain at least some, although not all, features of the reports. We highlight certain features that appear to leave open the possibility of unusual physical processes
other clips from his investigation linked above
"His initial impression was that he could be seeing sunlight reflected from large vinery glasshouses on the Island of Guernsey, tens of miles away. He had often seen such an effect before, which would vanish in moments as the aircraft moved through the critical angle for reflection. But this light did not disappear"
"There was still a primary contact on the left of the Trislander, said Kelly [air traffic controller], now 10 o’clock at a range of 3 miles, but nothing that appeared to correspond to the visual UAP. Kelly thought the contacts might be false echoes caused by anomalous propagation.3"
"Both witnesses disputed Capt. Bowyer’s later public opinion (based on a revised impression of range) that the objects might have been thousands of feet across. Kate had no definite impression of size, but felt that they were ‘‘nothing like as large’’, the nearest seeming to be perhaps 10 miles away, between the plane and Alderney. John had the impression that the object he saw might have been smaller than the Channel merchant vessels they saw during the flight. In other words, their visual judgments at the time were not dissimilar to Capt. Bowyer’s"
"In summary, the radar evidence examined is not helpful in establishing the presence of unusual phenomena. An ATC radar echo reported below the approximate visually-estimated position of one UAP may have been associated with an identifiable moving surface vessel, which may also have been picked up on the low beam of the weather radar. At the same time there are factors—use of MTI and poor sample rate—which limit the usefulness of this negative result, and pending the results of the GEIPAN study the ATC data files have not yet been examined to a level of detail that would completely rule out the presence of interesting primary echoes."
"But we also noted the coincidence that the average LOS from the Trislander to the lights was not far from the sun azimuth, inviting a reflection theory. Some witness descriptions used phrases like ‘‘sunshine yellow’’ and ‘‘sunlight coloured’’, which were also suggestive. And if Patterson could not see anything at 1413UTC from above FL65 and reciprocal to the Trislander’s LOS, why not? A FlyBe 146 in a similar position a little later also reported no visual contact. An optical theory might explain these anomalies. An optical theory would also be consistent with the absence of unambiguous radar evidence"