Surely she died much earlier that day?
BBC suspended programming at around 12.30. Why would they do that if she was ill - even if she was seriously ill or had a stroke or something? They wouldn't have been able to know how long she had left and potentially it could have all gone on for days. So they must have known she'd passed.
Also Huw Edwards travelled in from home (bringing his black tie with him). Why would he have done this unless London Bridge was in action. Again, if she was still alive but on her deathbed it could have lasted for days.
Why did the MP's leave the commons during the very important session on the energy crisis? Again if she was only ill they could have continued so the way the message was phrased must have triggered that response- therefore she must have died before that time.
I suspect she died early that morning, or even during the night before, but the Palace knew how important Truss's announcement was to people worried about the energy crisis and so they sensitively stalled on any news until the earliest possible opportunity (knowing that Parliament would have to shut down) and that moment was 20 seconds after Liz Truss had sat down after giving her announcement.