I'm pretty sure this is classified as a mistorical. I am sure the author got the use of titles and names wrong. (Just the St. Aldwyns, thankfully—that I noticed—but that was startling enough.)
As for the mistorical parts: The number of pistols and shootouts would have been more in place in the Wild West. That stood out for me. This is England we're talking about!
Now, I can't speak for the mechanical contraptions as I'm vague about the inventions of this time in history, but I'm pretty sure that some machines and tools in the book might be a little ahead of their time. I'm okay with that.
All that aside, I found the book to be fun, adventurous—even romp-y, different and very entertaining. I enjoyed the relationship between the h/h and I very much liked many of the secondary characters.