Well, you see, long ago in a far away land, called England, there was this writer, his name was Bill. He had a way with writing tragic comedies, tragedies and historical tales that usually ended in death for quite a few of the main characters. These people, known as the English, they tended to get a kick out of this. As far as I can tell, they figured, if it ain't broke, don't fix it.
(Trying to be humorous here, not condescending)
But, really if you look culturally, I think there is a lot of truth to this. England as a whole has no problem embracing death as a central plot theme, and the idea that NO ONE is static in a series. (You ever watch Dr. Who? - that show is built on the platform of if the star wants out, we don't need him.)
Meanwhile across the pond, we Americans are still arguing over why Aston Kutcher should have/should not have replaced Charlie Sheen and that it was all the studio's fault that they wouldn't bend over backward and kiss CS's backside. Frankly, I'm all for the BBC, kill em' all, let the Nielson's sort em out.