The experience of England after Roman rule collapsed was unique, and Harper glosses over this.
You've read the book, so you must know that the whole of the first section is devoted to showing that the English experience was completely un-unique i.e. it was overrun be foreign-speaking barbarians as soon as the Roman Empire collapsed exactly as happened in Italy, France, Spain and Portugal.
The English, according to orthhodoxy, were then unique in having their language changed for them by these barbarians (as Italy, France, Spain and Portugal didn't) but I don't expect you'll want to be using that.
You're fully entitled to question what I make of this situation but you can't say I gloss over something that I actually trumpet from the rooftops.
Accordingly, I invite you formally to withdraw the charge.