The author, although well-versed in the history and social mores of the post-Roman-Occupation of Britain, has chosen to divorce her characters and plot from that specific time-stream and create instead a kind of parallel world based on those real-world features. As such, I kept expecting some sort of fantasy trope - magic, strange gods, epic heroes - to intrude. Instead, it all reads as a superior sort of historical novel - albeit not the history we know.
It's a strange departure from the generic norms we're accustomed to, but it does give the author absolute freedom to construct characters and plot while still using a relatively familiar landscape: it's post-Roman and, judging by the names, post-Saxon invasion.
I was initially doubtful, but, as I became more involved with the story, it began to seem no more unlikely than the host of Arthurian novels I've read, and the protagonists were well-rounded and engaging. I'm now looking forward to the second volume in the series.