Don't get me wrong I like Harry Potter. But it is hardly some sort of renaissance of literary art. It is the intertwining of two very British literary traditions. The coming of age story and Arthurian legend. If it seems hard to believe think of it this way. Take all the wands, wizards, talking paintings and faeries (the fantasy elements) and you end up with a story of a public school coming of age novel. Probably set from 1938-1945. You take away the public school coming of age and you get a predestined hero being helped by a group of friends to overcome supernatural evil, Arthurian legend. I know that I am oversimplifying but there is no element of the story that is unique in its own right other than the combining of the two stories...which in itself wasn't really original because other authors had done it before see "The Worst Witch" by Jill Murphy. Was Harry Potter good? Yeah, I think it is very entertaining. Is it literature? No. But then again I think a lot of what counts a literature is worse than Rowling. And don't pretend that the bump in literacy is anything other than the by-product of mass consumerism. Think of all the good we could do if we treated scientists like we treat Bill Nye.
You're arguing that Harry Potter owes a lot to TH White. Sure, obviously. That doesn't make it derivative of The Once And Future King. "Derivative" means, at its most basic, "imitative in a negative way." Having written 500 words or more on how Harry Potter is not the most amazing new thing under the sun, I think we can stipulate that JK Rowling owes a lot to basic literary tradition. But that's a long damn way from being "derivative pap."