Just had a thought, wondering about American readability of “cheap plastic carrier bag”: use Google Books to search Elizabeth George (mystery writer) for the phrase. In fact: she uses “shopping carrier bag” on page 26 in Something to Hide (Google Books)
So, yes. Since she writes with Britishisms for Americans, her books are probably a good resource for these kinds of questions.
I’ve learned British expressions through context, either through British writing or your many wonderful TV series. So, “carrier bag” isn’t a term I’d use naturally, but in the context I’ve understood it as what we call a grocery bag. Same with “car park,” “biscuit,” and other Britishisms. Oh, and “crisps”! I’m also a bit of an anglophile, so.
The nuance of your “quite,” though, is still a puzzle: apparently “quite nice” is more pejorative than we Americans understand it? More like, “it’s ok…”? (Though tone tends to signal whether “ok” means “it’s all good” or “mmm, could be better.”)
Especially at the drafting or early macro-scale revising, I wouldn’t fret too much about specifics like whether Yanks or others would grasp each expression quickly. For my part, I enjoy seeing expressions that are new to me. Probably like Americanisms y’all read: they add a bit of foreign mystique.
Note about not letting this stop your flow in writing (and note to self, since I face questions like yours myself): might be best to keep going and consider readability for non-British readers at the later draft stage? I’m highly practiced at stopping over questions like these, when continuing is really the most practical path. I’ll have to make a pass or two in the micro-edit stage about language (my case: does it mimic the century/place enough, or is it too modern?). (And, huguatrix, why don’t you listen to this wiser voice, hmm?)
Sad note: I started to scan this at the beginning and saw vic-k’s name. I’ll raise a glass to his joyous mischief-raising in the skies. 
