I had read previously that the then Prince Charles was visiting her every day from Birkhall, where he was staying. When she started to decline, Charles and Camilla were about to host a reception somewhere, so all those attendees, must gave been aware, that something was up.
They wanted to get Edward, Andrew, Sophie and William there asap, so pretty obvious when they all flew from RAF Northolt (I think).
Then they wanted to inform family members before the press. Lady Colin Campbell has stated the news was released on one or two newspaper websites, but then withdrawn. However, BBC presenters started changing into dark suits and ties. But the government (and obviously as we saw some opposition members) were informed of her sudden decline. I imagine that some Commonwealth countries also had to be informed before the announcement.
So, I guess too many people were aware, before the official press release.