I disagree. The people making the videos are not in the US or UK. Furthermore, the people making the video are already breaking US and UK laws -- murder. The simple act of making a video (of some kind), posting it, linking to it, nor viewing it are illegal in themselves. However, if the government of the US or UK deem the video content to be "extremist" content, now it becomes illegal to link-to, or view the video? That seems bad to me (censorship). (again, murder is already illegal). Where does it draw the line? What about extremist videos that don't include murder but talk poorly about the US and UK governments and people? Will be deem those to be illegal to view too?
Police can already arrest the people making the videos (if they were in the UK or US) without having to make it a crime to simply view the video.
I don't buy the argument that these videos are propaganda to recruit -- they are there to stir fear and incite the idea that no one will save you if you are captured. The goal of the videos are to bend the public opinion.
The video will only sicken the majority of US and UK citizens -- which will in turn demand direct military action. This is, what both administrations (US and UK) have been trying to avoid recently as both have campaigned in large to avoid any direct use of the military (aka, boots-on-the-ground). It would be bad, politically, to then renege on that and send troops in.