We get to see in detail what difference a powerful magician makes in a war, so if there is practical magic in the rest of the world how in the hell did England keep its colonies after they lost theirs? But if there's only ever been practical magic of this strength in England, how did they not conquer all of Europe yet?
For reasons of my own I find this really interesting! Probably, as you suggest, just an artifact of where the boundaries on the story the author wanted to tell were, but there's so much interesting world-building that could be done...
(Also enjoyed reading your review in general; I tried reading the book in question and couldn't get through it a while back, but you make it sound worth a try.)