I could easily see someone caring about capturing Fillion's speech patterns but not caring about using accurate New York City slang.
Even if I understood how you could think such a thing was plausible, it just doesn't make sense in the context of the actual books. The level of accuracy and detail in the books' portrayal of New York City hugely surpasses that on the show itself.
Besides, I never said anything about New York City slang. I'm talking about British slang that, to the best of my knowledge, nobody
native to America uses, like "loo" or "lav" for bathroom or "liner" for garbage bag. Stuff that I wouldn't expect any American writer to use in their own everyday conversations and therefore would be surprised to see them use when writing American characters.