[Despite himself, as soon as he hears the name, Jack smiles. Even if being displaced from time and space remains a daily exercise in mild disorientation, there is a strange comfort in hearing one fellow English voice after another, from worlds where autos still run, and where "The Big Smoke" is not universally understood to mean London shortly after the fall of society.]

Ah, now that's the question that gets you nothing but theories. Two weeks in and I still don't have the faintest idea - and as I understand it, two years isn't much better.