Oh, I totally agree, Caz. Those blunders can really take a reader out of the story, especially if the reader is actually from the UK! I’ve said it once, and I’ve said it before: how hard would it be for HR and CR editors to keep a laminated cheat sheet of titles and Britishisms to check submitters’ work against? Heck, Harlequin should do a blog post on this topic and provide a permanent link to it on their HR submission guidelines page.
Funny side note on this topic: There were some complaints on Amazon that Fifty Shades of Grey, which allegedly takes place in Seattle, WA, USA, is littered with Britishisms instead of Americanisms. It made me think of all the blunders in Regency HR you mentioned. I thought, “Yep. What goes around, comes around.”