Agreed, though I believe the digression is deliberate. This isn't absolutely clear from the cover blurb but it is kind of suggested. My problem with the digression is that it reads to me like padding. The author doesn't have enough to say about, or doesn't know enough about, the actual thing or place she's supposed to be discussing, so she gives the reader a bit of Virgil or something instead. But anyone interested enough in the Romans and/or Roman Britain is likely to have already read Virgil.
Now, those interested in the Romans (Republic and Empire) and/or Roman Britain are also likely to have read Tacitus. But it's worth repeating Tacitus because he wrote about Britannia.