How does English Magic work in Jonathan Strange and Mr Norrell?

Blogger "seriouslypleasedropit" has joined-together the textual clues, and came up with what seems like the true explanation (slightly edited):

The eponymous Mr. Norrell is a somewhat timid creature—a landed country squire, ever a bachelor, who likes nothing better than sitting in a corner reading a book, and is likely to describe anything that disturbs his study as “irregular.” Through years of study (and a near-limitless inheritance with which to buy books) he has reconstructed a system of magic which allows him to perform various feats. Norrell’s magic is effective, but wooden; he avoids risk, and is very concerned with magic’s reputation in Society, as it has hitherto been disreputable...

In contrast is the other protagonist of the book, Jonathan Strange, who, having devoured tales of the Old Magic, wants to duplicate (and improve upon!) those ancient feats, and is willing to consort with dangerous fairies to do so. ...

Mr. Norrell and Mr. Strange spend a good amount of dialogue... as they argue about what magic ought to be. Norrell wants it to be practical, controlled, systematic; Strange wants it to be wild, wondrous, and spirited. Norrell wants nothing to do with fairies; Strange knows that the greatest English magician ever, the “Raven King,” heavily consorted with them, and wants to follow in his footsteps.

The book is long, and much happens, but eventually Strange finds the secret of what magic is, at least within the world of the book: 

 “It is not so hard as we have supposed. Tell them to read what is written in the sky. Tell them to ask the rain! All of [the Raven King’]s old alliances are still in place. I am sending messengers to remind the stones and the sky and the rain of their ancient promises.”

The gist is: long ago, the Raven King, having learned from the fairies to speak to the stone, the trees, and water of England, made alliance with them on behalf of all English magicians. That was the source of the magic, and Norrell’s formulaic magic is akin to a child writing letters to distant servants in the hand of a long-dead father, thinking they are spells rather than messages.

Excellent! I would add that this also explains why, when the Raven King departed from Northern England about 1400, English magic faded and failed. The alliances he had made with the 'nature spirits' of England stopped being renewed and gradually lapsed.