Honestly, loads of bonkers conspiracies!
The medical team knew around midday that she was on her way out so the family were called, the prime minister informed, and Operation London Bridge/Unicorn was started. There was very likely a part of Operation London Bridge on what happens during the Queen's last hours if she's unconscious and unlikely to fully recover, and it probably includes announcing a subtle message to the public so they can be prepared/explain the unexpected movement of other royals, hence the rolling BBC coverage, and to start moving soldiers etc to where they need to be.
Apparently, the Royal Family often announce big news around 3pm, so I'm guessing the BBC was expecting an update for then so they got the newscasters ready in black ties, but then nothing. (They mucked up there)
Their grave faces at this time were the realisation that they would have to waffle on about nothing for another 3 hours. 😅
Anyone who says they knew before 3.10pm was either trying to be clever and making an educated guess based on Operation London Bridge being actioned, or is a nutter. Anyone who knew between 3.10-4.30pm when the PM was informed isn't going to be telling Mumsnetters. Anyone who knew after Liz Truss isn't that special.
RIP HRH