It’s in the bit about the the Ollivander Family’s history as wand-makers in Britain; chapter 5, I think. They arrived with the Romans, and upon realizing that the native British wizards weren’t good at wand-making, they set up a booth and stayed there.
Time to write historically accurate fanfiction about the relationship between Ancient British wizards and Ancient Roman wizards in the time of Boudicca’s revolt, right?
(If the Mitfords post did not tip you off, it should be clear by now that I am a huge Harry Potter geek when I am when I am not blogging about history)