The thing about knives. The Doctor needs to learn that knives unlike guns can be used in peaceful ways. Knives are very useful tools at times, that is why we all have them in our kitchen. There is nothing wrong with carrying a knife. Just ask Macgyver.
Hmm, that video talks about knives the way we talk about guns.
Perhaps in the current climate in the U.K., it does makes some sense for the Doctor to say what she said.