Good question, I'm still not sure where the original Arthur is. The easiest and funnest thing to say is that he's with Colin, wreaking havoc on the modern world.

I think if I were going to continue this story, I might have Bradley trying to make the same decisions Arthur did in the legends, even if they were bad decisions, because he thinks he must preserve this Arthurian legacy. Like, he doesn't try to escape his fate by turning Lancelot away or avoiding Camlann, he sees it as his duty to keep fate running clockwork. And the legend becomes a self-fulfilling prophecy.