I've often cracked, when naming characters in my novels, that the Big Book of Regency Baby Names is actually a slender pamphlet. And now I've got actual data to back that up. I'm reading a book called The British Army against Napoleon: Facts, Lists, and Trivia 1805-1815, by Robert Burnham and Ron McGuigan. It's pretty much as dry as it sounds, though the trivia interspersed with the lists is fun, and it's full of useful info to someone like me who's currently writing a novella set in the aftermath of Salamanca and a novel spanning Bussaco through Fuentes de Onoro.
The chapter on the life of an officer includes a list of the fifteen most common first names among British officers serving in the Peninsular War. They were:
Just shy of three quarters of all the officers serving had one of those names, and over a quarter were named John or William.
As for their last names, here are the ten most common:
Very Scottish, and I'm betting I've got some extremely distant cousins among their number. (Yes, Fraser is a pen name, but it's on my family tree--the name I pull out at Highland gatherings whenever I run into the kind of people who want to check your pedigree before selling you a tartan scarf, and, yes, I really have met a few.)
I'd love to see an equivalent first name list for the wives and sisters of those officers. I'm guessing it'd include Jane, Catherine, Anne, Charlotte, Caroline, Sarah, Frances, and Mary for sure, but I'm not sure what would round out the top 15. Eleanor? Harriet? Lucy? I know Georgiana and Cassandra were in use (obviously--see the Duchess of Devonshire and Cassandra Austen), but I don't think they were that ubiquitous.
I try for realism in character names. I wouldn't limit myself to just those top 15--though TSL has William and AMOI James--but I'm name nerd enough to check the etymology of a name to make sure it's not too modern and sensitive enough to social nuances that I'm not going to give an English character a blatantly Celtic name, and if I picked anything really unusual I'd be sure to justify in my own mind, if not on the page itself, why that character's parents ventured off the John/William/James/Jane/Catherine/Anne beaten path.