PREVENT is pretty dire, though.
I agree it's a hard problem, but getting it wrong is going to lead to the Northern Ireland situation: overreaction by authorities creating political prisoners who will be martyrs to their cause. Bobby Sands is still a hero to many.
> there is nothing effectively you can do because no law has been broken until there's evidence that they start assembling the components for the bomb or whatever
Sure, but that is basically where we as a society have agreed the line is drawn for decades. Otherwise you start prosecuting people for "sedition". And it'll inevitably get used against peaceful leftwing groups and not against radical xenophobes, etc.