We don't have to suppose. This has happened in recent history. When a small group of british people turn hostile and violent for a specific cause, the media services and the population decry their actions, and the British government invariably arrests them. Thatnks to football hooligans, riots, the IRA, 7/7, and its nanny state system of CCTV cameras, the UK is actually quite good at this sort of thing.

In comparison the islamic world tends to take a 'boys will be boys' attitude to this kind of thing. While I appreciate the utility of avoiding words like 'blaim' and 'fault' it's kinda hard when the 'victims' are not only indirectly supporting terrorism but actively egging them on.