The difference is that being a member of National Action or another similar group is illegal.

Someone who is radicalized at a mosque won't be able to join a terrorist group without leaving so they are referred to Prevent. Strangely, the ones they catch on the far-right are overwhelmingly young too, they claim this guy was high up...19, member for 2 years, 40 conversations with other members. They seem to target kids who may not know the legality of the organization (or you can join a terrorist group like Begum, be in charge of the "religious police" and the BBC will give you a podcast).

The reporting - for example, including Hope Not Hate's role in stopping a plot to kill an MP...not the police? - is bizarre. The article says one of their crimes was celebrating Jo Cox (this guy was 11 years old when that happened)...as we know, the response to Amess on the UK subreddits was mixed. Brain on politics, what a drug.