@G-BLEW By "strong" I actually meant "convincing" rather than "Listen up, bucko..."
@bogopper, in this environment (sadly one I paddle in too often for anybody's good) it seems likely more to reach an acceptable solution by speaking the same language and asking them to think about what they are saying.
One of the things I am (re-)asking is how they define "clearance" (CEMA 1979) given they say "nothing has changed" and there's no explicit permission required today.
You are in effect "cleared" once you notify appropriately, leave from an approved airfield and aren't then challenged within the notice period. That should be the same from January, I say in my draft.
The risk of asking a question in the wrong way is that they dig in their bureaucratic heels and compound what I reckon are their existing errors.
As for "how long do you wait?" - so far as I'm concerned that question already has a clear answer just as it does today ("until after your GAR ETA has elapsed") hence there's no incentive even to ask the question.