And why the UK government? What about giving that same power to the government of Afghanistan for example?
From a utilitarian perspective, they could use that "superflagger status" to prevent the spread of videos that have lead to violence in the past.
One simple example : http://www.nydailynews.com/news/world/google-remove-riot-spa...
However, from a freedom of speech perspective all that is very very wrong. If watching a video cause you urges to start rioting and harm people, maybe it's not the video that's the problem.