^Well, Buffy justified it by situating Sunnydale on the Hellmouth, thus making it a nexus of supernatural forces. But when Angel came along and had just as much supernatural stuff happening in LA, that weakened the premise somewhat. But really, if there are naturally occurring ruptures in the spacetime continuum, there's no particular reason why they should be limited even to Earth, let alone England. But then, the anomalies have always been handled a bit awkwardly. I mean, why is it that any time an anomaly appears in the distant past, some kind of prehistoric beastie just happens to go through it within moments? If anything, I'd expect most animals would run away from something that strange, or that if they stumbled through it by accident, they'd immediately recoil from the unfamiliar environment they ended up in and retreat back through it.