I'm not familiar with the specifics of what's in place in the United Kingdom. Certainly, the seizure of terrorist propaganda and the criminalization of advocacy or promotion of terrorism are important things for groups that don't fit the traditional terrorist organization designation and for individuals who don't operate within that framework. If a message from a group like the Muslim Brotherhood were to veer into that territory, it's an important way to deal with that and to stop it. There is recourse.
It may be difficult, if not impossible, to list an organization such as the Muslim Brotherhood as a terrorist entity, in that they may not meet the criteria, but certainly, if they are engaged in activities that are considered terrorism offences here, such as the promotion of terrorist activities, or if they are disseminating terrorist propaganda, these tools are in place specifically to stop that.