But a writer portraying characters who are native New Yorkers would not put British slang in their mouths just because he "liked" it, because that would be inauthentic.
Unless he just doesn't care about being authentic.
Try a little deductive reasoning, in the spirit of detective stories. Remember what I just said about the writer capturing Castle's/Fillion's voice exactly? Someone with that good an ear for distinctive character voices wouldn't simply "not care" about authentic speech patterns. There's also the fact that the characters on the show don't
use British idioms like that, which at least rules out Marlowe. As for Straw, he seems to have gone from LA to NYC without any intervening time living in the UK. (Although with a name like that, it's possible his family is from Britain, though I can't find any evidence of that.) So it remains an anomaly.
I wonder how long it'll be before the ghostwriters finally "come out" officially. Probably they'll keep up the conceit of Castle's authorship as long as the show remains on the air, and maybe for a while thereafter, what with DVD sales and all. I wonder if there's even a nondisclosure clause in their contracts.