The official position of the MOD is the UFO or UAP as they call it, have no defence significance.
The official records that have been released are supposed to end their interest in the UFO. These records were classified as unclassified, restricted and secret. It is interesting that the documents released are scattered for the most part and are of a certain level of intelligence. Any document that is released is only going to be released if it is expectable to the mainstream in general, of course there are exceptions.
DI55 (Defence Intelligence), contrary to the official position, were not the only departments to be interested in the UFO and the events that surround their encounters. The MOD had and still has highly compartmented divisions who amongst other matters analyze military and civilian UFO encounters. These are those encounters which are of a far higher quality of intelligence than DI55 had access to, and was highly compartmented. DI55 can only made judgements based upon the information it receives, and it didn’t receive all the reports.
GCHQ does indeed have UFO related reports which have been intercepted from foreign and domestic transmissions; these are considered highly sensitive, since some of the reports involve loss of aircraft. MI6 has some involvement has well, since they are interested in how other nations response to the UFO beyond what their official stance is, which is usually for mass consumption. If lost of aircraft is involved in the interception of these objects this is certainly of interest to the security establishments of the United Kingdom.
The Chief of Defence Staff is aware of the origin of the UFO and is officially informed. Many reports at the Top Secret level do exist; there are even records beyond this classification. One of the highest classifications in the United Kingdom is Deep Purple.
From the public’s view point we can argue that any UFO reported by a pilot (military or civilian) will be investigated from the point of view of air safety, and we would only know if there is a defence threat if the MOD investigated it. If an unknown blip on radar is detected and performs amazing manoeuvres, it will be investigated, since some could be the result of foreign experimental aircraft or drones.
So we can be assured investigations have never ended, it was a public relations exercise, based upon the reactions of the politicians and the public, that eventually closed the officially acknowledged investigations by the MOD.