"Diplomatic Clearances"
As Dep Def Attache somewhere really nice I used to submit DipClears on behalf of ASCOT Ops and others. As I recall the reg no of the a/c was not important (airframes get changed) it was the Flight Number/callsign which was key.
My host nation often picked us up for using 'ASCOT xxxx' and 'RR xxxx' interchangeably.
Batco