Far Right Wing extremist individuals foment conspiracy theories, and hatred on-line and elsewhere, and they constitute a "group" or "movement" even if studied individually they seem to be Lone Wolves.
The Security Services here in the U.K have taken over responsibility for monitoring these people, viewing them as a very real Terrorist threat. The U.S.A has been complacent for too long about this.
Potentially violent Extremists of whatever stripe must be identified and stopped, also racist comments and ideas must be called out whoever utters them, before the violence happens, many words are spoken.
What are the conspiracy theories? I don't live in London, but I've seen what's happened there, in France, etc. The no go zones, etc. Are those conspiracy theories, or is it that people are complaining about what is happening, telling hard truths? And hard truths must be silenced because they are expected to bend over and take it in the ass and to like it too?
Things can be twisted and make a false narrative, those can be used to create hate and fear. And cold hard truths, not exaggerated can also do the same. The first can and should be addressed. The second, if the perceived solution is to silence it, will only make it steam up worse and blow hard when it does. In the case of things with muslims and mass immigration, it's the 2nd one. An example of the first one, twisting facts to create a false narrative, would be American police systematically targeting blacks because of the color of their skin and killing them any chance they get. Looking at the true facts, numbers, etc, that is a twisted and false narrative that creates fear and started a chain of violence against police. But mass immigration, no go zones, the crime, the numbers, statistics, all true, and if people airing it is creating problems, then the problems need to be addressed, not silenced, and the longer it's ignored, the worse it will get. Better integration, less mass immigration, practical solutions can diffuse and lessen the problems. If politicians just ignore, mass immigration, and silence those complaining, the problems will grow and it will hit a real bad boiling point.