The Utøya massacre / bombing https://en.m.wikipedia.org/wiki/Utøya
Jo Cox UK MP assassination: https://www.nytimes.com/2016/06/17/world/europe/jo-cox-briti...
UK right-wing riots: https://www.mediamatters.org/sean-hannity/right-wing-media-u...
Attacks on immigrant vendors in Greece: https://www.reuters.com/article/us-greece-crisis-dawn/specia...
Dresden anti-islamist marches: https://www.theguardian.com/world/2014/dec/15/dresden-police...
I was in London at the time. They were sparked by the police killing of a drug dealer, rightly or wrongly, and were more chaotic and violent than any sort of political.
Whatever the reaction was in the US, that article just calls it wrong.
--edit-- I think it might be you that's read it wrong, the article is just about US right wing media interests using the London riots to further their political goals in the US.
You might want to take that link off your list, it doesn't support your argument.
https://en.wikipedia.org/wiki/Charlie_Hebdo_shooting https://en.wikipedia.org/wiki/November_2015_Paris_attacks https://en.wikipedia.org/wiki/2016_Berlin_truck_attack https://en.wikipedia.org/wiki/Manchester_Arena_bombing https://en.wikipedia.org/wiki/2017_Stockholm_truck_attack https://en.wikipedia.org/wiki/2018_Strasbourg_attack
If you meant nativist extremism, I suspect multi-party democracies allow something of a safety valve. AfD and Brexit Party voters can still feel like they’re being heard while identifying as less extreme than NDP or BNP. In the American two-party system, one side is always painting the other as their most extreme example with no credible third parties for contrast, and indeed extremists do sometimes rise to prominence from within either party.