See, Alfred is so important in English history not just because he created the entire idea of an English state, significant as that is. But he did three things of service to history:
- He put in place the foundations of a kingdom that spread across all of England (his grandson finished the job)
- He ordered that everything of importance be written down
- His legacy survived- no invader came along and burnt it all down, as kept happening in earlier times.
These three things mean that we have a wealth of written and archaeological history that stretches across all of England from Alfred's time onwards. We know everything that historians could reasonably expect to know from that period. In contrast to the preceding centuries, we go from a long period of 'we know almost nothing' to an everlasting state of 'we know nearly everything'.
Hence 'Dark Ages' has a particular pull in Britain, as for that period it's very apt. It's also where the basis of the Arthurian legend comes from, which gives an aura of cool.