That word is informal even in American English. It's less common in British English, but it's growing in popularity in both versions, and I wouldn't call it "not a word" even in British English. But I disagree with its inclusion in a first lesson, because its main use over the more common standard alternative (rot13: "fbzrjurer") is signaling casual speech.
Google Ngram Viewer lets you compare popularity of words in British vs. American English, so it's useful for investigating this.
British English speaker here - I'm having difficulty identifying the word you're referring to here. Or are you referring more to definition and statistical presence in common usage?
Since "gotten" isn't in the list, I'm going with "someplace". It even seems unusual to me as an American, as I would write either "some place" or "somewhere". If the phrase is also unusual in British English, that's definitely something I had never noticed.
Re:Multi-Layer Dictionary (2016)
by quietbritishjim on Tuesday July 23, 2019 @10:39AM
(#20507052)
Is "gotten" the only word that exists in American English but not British English (apart from "someplace")? Your comment strikes me as trying to bring up a pet hate in an unrelated conversation.
I'm sure it's not the only one, but it's the one that immediately came to mind. I thought it might be in the list because it's close to a foundational word in American English.
Sorry that you chose to read my comment as hatred. Yeah, that's not a real apology.
Thank you. Appreciated.
I know the word you mean (no spoiler tag, so I won't say it, just 'it could almost be German' {to use a linguistic stereotype}) but I wouldn't recognise it as _not_ en-gb, just unusual.
I'm en-gb native.
(Why do I mention that? Firstly, perhaps it's a fun puzzle for non-native speakers of American English to identify the word. Secondly, it's surprising that a difference between British and US English is apparent in such a short list of such basic words, considering that sometimes it's possible to write whole paragraphs of English without it being apparent which variety of English is being used.)