Not true. I presume that PSNI, like any law enforcement agency in the U.S., operates with some form of a use of force continuum that ranges from officer presence to deadly force. One of the most important aspects of that continuum that is taught to officers is that you can move up and down the continuum as the situation dictates. If you draw your sidearm you are not obligated to remain at that level of the use of force continuum. You should continue to make every effort to gain control of the situation, and deescalate.
At the same time, you are not obligated to use a lower level of force than that which you are facing, and a kitchen knife is a deadly weapon. It's very much a training issue as much as anything to educate officers in deescalation techniques, less-lethal weapons and control techniques, and scene management to minimize casualties. In the situation you describe though, it was absolutely appropriate for her to draw her sidearm, while still doing everything in her power to regain control of the situation so that she would not need to use it.