What I didn't think of till much later was that there's a glaring example in British history that no one mentioned: namely, the successful usurpation that was the Glorious Revolution and the subsequent stories of the Old and Young Pretenders. There the "official" narrative has had to be that William's usurpation was indeed a Good Thing, and the Hanoverian succession too, while the Jacobite "true heirs" have to be made illegitimate - a narrative that could only be maintained (and that barely) by affirming Parliament's sovereignty over the monarch. (For those of us who prefer democracy to monarchy that's quite a sop, of course.)
Not that this has stopped romance attaching to the Jacobite cause - obviously. But even before Scott turned his genius to the matter in Waverley it was romantic as a lost cause, and it was increasingly in the lostness that its value resided.
Still, while William III has glamour for some on religious grounds that may be said to trump national ones, there are few less charismatic runs of monarchs than Anne and Georges I and II. Despite the crucial events of their reigns, still so relevant to the world as we inhabit it, to say nothing of the Revolution itself, they are muted in our National Story. Many young children learn the names of Henry VIII's six wives, but what do they know of Walpole or the Treaty of Utrecht? I feel fairly confident in saying that in 99% of cases the answer is "Nothing". I've often wondered why this was the case, but now I'm thinking that it may be because stories of "good" usurpers run counter to the narrative conventions that we assent to. There's a cognitive dissonance, and our eye slides over it rather than acknowledge the discomfort.
Joan Aiken, of course, in writing her Dido Twite books, had to imagine a Britain where history proceeded on conventional narrative principles, with the "good" Stuarts still in power and the "bad" Hanoverians as unsuccessful usurpers.