If the Senior had stuck her head through the door and just said quite simply, "Captain, I'm going to start an evacuation." That would have been enough to get the Flight Decks attention. From the AAIB report she hardly tried to tell them, it sounds like she made her own decision and that was that. Told the rest of the Cabin Crew and off they went.
A quick sentence and that would have given enough time for the flight crew to say either "NO! STOP! We have it under control!" or "Ok, if you think it's that serious, take your station and we'll commence it, under my authority."
Once again, lack of communication, from both sides of the door have led to an event which COULD have led to more serious injuries. SEP refresher training for all I think.