BOAC
The point of the post should have been apparent in the first sentence--do not descend without knowing the terrain and your clearance. The OP was in EXACTLY the same position--direct, off-route to a fix except he only had FL80 as a level clearance, no specific descent clearance from ATC and no published route. My two muppets had an altitude clearance that was NOT appropriate for the terrain or distance from the field, but acted on that clearance, wrongly but the controller didn't give them any help.
having declared this 'leaving' to ATC - who did not issue any further instruction - I reckon that ticked all the boxes - safe and efficient.
My guys did 'declare' leaving and did get an acknowledgment, hardly a safe and efficient.
Given that clearance to 4,000 feet by the controller, how was a crew supposed to know it would not be safe to descend? It was a bad clearance on the part of ATCO and bad airmanship on the part of the pilots.
As Intruder and I have been saying, published route or maintain your altitude or level until on one.
My two were summarily unqualified--one never to be an commander.
If my last sentence is "ridiculous", it is only because it rules out ANY descent when off-route which is a bit extreme, I suppose.
GF