Hi Matthew
You are presumably aware that some years ago the MoD released/declassified a UFO report in which the only major redaction was one entry on a list of possible US aircraft that could be mistaken for extra terrestrial craft. From memory, the list included the SR-71 and F-117.
You are also presumably aware that there are RAF aircrew who have seen unidentified aircraft (not aliens) in UK airspace, at least one of whom mentions it in his autobiography.
It might be helpful to FOIA for the redacted identity of the aircraft in the report. This might have absolutely nothing to do with do with the Calvine incident, but as others have indicated, there is much we don't know about "black" aerospace programmes from that time, and a 'worldly' explanation is much more likely that an 'other-worldy' one.