Britpick

To Britpick, verb. The act of noting and complaining about any and all instances of non-English English, in particular the so-called 'American English'. Britpicking agents deny this dialect the status of a true language, and are wilfully confused by it. Occasionally, they are also overwhelmed by it; overly American fics have been known to cause stress, headaches, stomach upsets, violent rages and fits of hyperbole.

Britpicking can also be requested of a beta by an American writer; it is particularly common in the Harry Potter fandom. Here it is more of a check to see that everything fits in with British culture and, yes, language. Particularly important in slash fics, a Britpicking beta ensures UK-based readers and agents can cope with talk of pants and the lack thereof without the danger of unscheduled hysterics.

Reverse-Britpicking may apply if a Brit ever condescends to write in an American style, but there isn't a fancy rhyme-based portmanteau for that, so it's talked of less.