Then I wondered if it were true.
British Bagel flavours I've encountered:
Cinnamon & raisin
British Terrorism flavours (not personally encountered, drawn from news).
(I think you can count both, even if unionism isn't really a terrorism on the mainland?)
animal rights extremists (I don't think they have actually killed anyone, but the most extreme actions *probably* count, as being designed to induce terror?)
Meibion Glyndwr (??? still current? Are Meibion Glyndwr still burning the odd holiday cottage? They definitely don't kill people though, not sure they count)
Total: 5, sort of.
I don't think Mebyon Kernow count, not even the ones that deface English Heritage signs. That's just graffiti.