It is an opinion that gets discriminated against a bit more than most (along with Anarchism) simply because the Media and the Police don't take either seriously as ideas.
Example: People planning republican protests during the Royal Wedding were arrested in advance and held during the event, to stop their protest being heard[1]. As far as I know, nobody was actually charged with anything, only arrested on suspicion of "conspiracy to cause a public nuisance" and then released when it was all over.
Nonetheless, writing about the idea is perfectly acceptable, and a protest in a less sensitive area / at a less sensitive time would be "tolerated" (not that this justifies the censorship it gets sometimes.)
[1] http://www.theguardian.com/uk/2011/may/01/wedding-activists-...