The question comes from a book, but it is an historical novel based on real events, so I don’t think Cafe Society is appropriate.
1843, Victoria has been Queen for maybe 6 years, and Robert Peel is the Prime Minister. One day, in the book, because Albert is away north somewhere, Peel is invited to accompany the Queen in a carriage ride around the city, as she was apparently wont to do sometimes. They have a conversation about some current political matters, and throughout this conversation he addresses her as “Your Highness” instead of “Your Majesty” or just “Ma’am” as I guess folks do today after the first time.
The author is a former college professor and presumably reasonably well educated, even though he is an American. It seems a stupid mistake to make, if it was such, so I wondered if, for some reason, she preferred this form of address at this time. I can’t find any reference to this practice in a couple of brief Google searches. One possibility I thought of, which seems far-fetched, is that she did not want to call attention to the fact that she outranked her beloved Albert, who was only styled as Prince, so while he was alive she went by “Highness.”
Is this close to the truth? Is it common knowledge in the UK, at least among those who study royals of the past, or is it an egregious error made by a stupid American, or something else? I would ask Lucy Worsley, but I don’t know her.