[ShinyBat]

I guess it depends in part on the definition of embargoed. 

Normally you can tell when a story has been embargoed as all outlets report it at the exactly the same time.  The BBC may have been forewarned that an announcement was coming, but not given an exact time when they could report it.

This is what I suspect.

Normally an embargo would end, as you say, at a very specific time. I used to work at ONS, and all the reports on a big ONS release (such as the GDP one that I helped with for a while) were dropped at 9:30am along with the actual data. Journalists get the early access to the data under embargo - no small deal considering their financial market implications - so that they can put together an article explaining what's happened without the need to wait for everyone to rush at 9:30 to write hurried interpretation. That's your usual "embargo" protocol - news withheld until an already-agreed moment, in that particular instance on a fixed repeating schedule, to smooth out the reporting.

Obviously, royal deaths aren't pre-scheduled. The purpose of an embargo for the death of the monarch would be to ensure the mechanics of the state - your Operation London Bridge protocols - are all ready to go. Black outfits for politicians and broadcasters. Social media ready to go. Flag-lowerers ready at all necessary locations. Goodness knows how many other details.

An embargo of that nature would presumably last until "whenever everyone's ready." In this instance, "when everyone's ready" was a very neat 18:30 BST. But we couldn't know that when Liz Truss heard the fateful code. So an open-ended embargo was required, and an open-ended embargo was what we got. (As we did for Diana, for mechanically similar but logistically different reasons - there was no constitutional process, Diana wasn't even a royal at all at that point, but the family had to be woken and informed before the public could find out.)