The public were told formally at 6:30 pm
The 'under medical supervision' announcement was at about 12:30, and presumably family were informed at around that time (possibly just before)
Then there were announcements made about the Sussexes intended travel (was it Scobie? I can't remember) so it seems pretty likely that he did know.
Actual death was certified at 3:10, so the death would have been close to that time - there would have been officials present to declare "The Queen is dead, long live the King" so not a lot of wiggle room for even a few private moments.
The PM was told at about 4:30
Was Harry really on a plane for about 3 hours that afternoon - from say about 3:15 to 6.15? Where was he going? It really doesn't take that long to fly London to Aberdeen.