I have a memory of only two places where we were cleared for a three engined ferry without referring back to HQ. One was Salalah and the other Aldegrove.
Both during the time of their respective 'troubles'. Is my memory playing tricks ?
Wasn't it in the Flying Order Book, my memory was both Salalah and Aldergrove as we were possible recipients of inbound ordnance?
We used to do a demo one on the OCU with one at flight idle if memory serves I think. That or PRT, can't remember