I think there's another explanation. Several of the Very Highly Paid Help at the top of Air Support Command and 38 Gp were keen on the nags. So it tickled their fancy, if you'll pardon the phrase, M'lud, to have 'their' aircraft going under an international callsign that reflected the Great British Social Occasion which none of them would want to miss. I'm a bit surprised they didn't insist on 'Royal Ascot'.
I think the exact meaning of the 'OT' came later, in post-event justification.
All very British, and redolent of a different era.....
airsound