So what you are saying is, that all the police have to do is label someone a Nazi, and you'll give them a free pass for whatever it is that they do?
I do think the police should act to try and prevent violent groups from committing violence. Your position seems to be that the state should accept violence as long as the people committing it claim to be political? If anything organised political violence is the type of violence that it's the most important to prevent because it has by far the largest potential for harm.
I can talk about specific things the UK police have done when targeting 'extremist' groups and how I disprove of them if you want (undercover police assuming fake identities to the extent of having children with women in the targeted groups being one). But disapproving of specific things isn't the same as saying the police shouldn't be investigating and infiltrating organisations advocating violent ideologies.
It is dangerous to believe "oh, I don't care what an oppressive government does in order to stop Nazis" because a whole lot of people are being called Nazis these days.
The same exact thing happened during the red scare. It was not JUST the revolutionary communists who were literally trying to overthrow the government that got oppressed. A whole lot of other people who definitely didn't deserve it, got oppressed to.
Human rights exist for everyone for a reason.
I would hope that describes virtually every post war western government.
And many of them have implemented various levels of 'authoritarian' systems to prevent the (re)rise of Nazis. Ranging from the banning of books in Germany to more mundane things like the British police spending a lot of time and money infiltrating neo-nazi groups and banning them. In the latter case exactly the kind of thing you are worried about happened - the powers granted to the police were also used to target other non-nazi groups like environmental protestors. Despite being very critical of police tactics in the latter cases, I still have no problem with the police targeting Nazis.
Anyway, western governments don't need to raise the spectre of Nazis to gain support for increased police and surveillance powers. They have plenty of more recent things to talk about (from Irish terrorists to ETA to Islamic terrorists). This comment thread is evidence enough for that - there wouldn't have been nearly the same emotional reaction to somebody talking about bombing ISIS.