I still don't understand why you find the accents an issue with this show . They went to Mars and lived through a few generations . Fine . Or went to the Belt and went though a few generations . Obviously it would have been a family or even a group of families going there . So , they all stick together and speak there launguage with THERE ACCENT'S because there sticking together . It happens here on earth as the Italians, Portugese , French, Cockney Brits, Canadian, American,Japanese,Chinese,Korean all live in the same area together keeping there dialect alive . So she is cockney. Why would it not be plausible that in her family and in there heritage , they kept the cockney accent alive and well, with her family lineage . 100% they would keep there accents as would all the other languages . You don't see Mr Mau speaking with an American accent . He has an Asian accent because he is Asian and proably speaks Mandarin yet I am sure his linage is Asian launguages, not specifically English or American or a UN accent . Bobby speaks with a New Zealand accent because her heritage is from New Zealand and as I stated before , good chance they migrated as a family and I bet her dad never ever lost his accent because of his parents and there parents .
I have friends from different countries with different accent's and they have been here in Canada for more than 50+ years ! They still have there British accent's . So I don't see how a 200 plus year span would cause such a switchup on how they speak other than switching it up with Creol belter banter but there original dialect would stay alive and well even with learning belter as Naomi demonstrates in The Expanse . Remember also that Naomi joined the OPA and is an engineer trained outside of the belt , not trained by belters .