Regarding the spelling question, for our standards here, it should be consistent throughout the whole fic. You don't just use American spellings for American characters and British spellings for Malcolm. On occasion, for our authors who are accustomed to writing British English, we have allowed British conventions to be used, as long as the entire fic is consistently done the same way throughout.
On the other hand, just like with spelling out Trip's accent, you don't want to go overboard with some of this other stuff just for the sake of making Malcolm "more British." I can't think of too many times when Malcolm's used British idioms, so I would question a fic where they seem conspicuously present. (Natch, I qualify that statement by saying that I've watched PBS and BBC America for years, so it's possible I'm so used to it I don't notice it as much any more.

) To me, that would be like Archer asking Trip when the engines are coming back online and having Trip answer something ridiculous like, "Don't you pay no nevermind, Cap'n. Ai'll get this sonomabitch runnin' faster'n a pissed off gator in no time a-tall."

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