Two things stick in my mind on this topic:
1. When the MoD declassified its 'UFO Guide' some years ago, it included a number of aircraft types that might explain the actual identity of a reported UFO. In the US section, it included obvious candidates such as the F-117 and B-2. But one aircraft type had been redacted in its entirety.
2. I'll need to dig out his name, but I read the autobiography of an RAF fast jet pilot a few years back, in which he stated he had seen something flying into RAF Mildenhall very late one Sunday night. It was triangular in shape (but not the B-2). I don't recall the details, but I think he was at that time on a ground tour, and had some kind of liaison responsibility between the RAF and USAF. He phoned Mildenhall the next day to enquire what the aircraft type was. They responded that there had been no aircraft movements that Sunday. He fairly rapidly then gets a call from someone in MoD telling him that he didn't see anything and to stop asking questions.