The Radio 4 version of this story is even less likely IMHO. They claim that QRA-South was phoned and told to expect an american to drop by, who would relieve the QRA pilots. They then say that the american duly arrived, and shortly afterwards flew a QRA lightning on a mission. The different versions of the story they told then claimed that said Lightning returned with or without some/all of its missiles.
...because of course the USAF had a plentiful supply of current, Lightning-qualified pilots and trhe RAF routinely handed over aircraft and stood-down QRA to foreign officers on the basis of a single phone call. Happens every day...
PDR