I was around at the time.The story was that the MU 'phoned the Transport squadrons asking if they could borrow a pilot for some ground runs. It was only when they got no takers that the MU boss elected to do it himself to avoid any more delays.
The aircraft was reportedly chocked and with grills over the engines and certainly had no hood or operating bang seat. It was an inadvertent selection of reheat on both engines that got the show on the road.
I seem to remember it was a Britannia crew (the aircraft not the airline) that was taking off on the main runway when the Lightning hurtled across their bows.
Apparently nothing was said by the pilots until they passed a thousand feet or so when they just looked at each other and said:
....errr?
The poor engineer Wg Cdr was reputed to be twitchy at every sound of a jet engine after that, even though one rumour was that he did it deliberately.