Oh man, I always laughed when I heard really posh people speak -- there were a fairly large number at the uni I went to. That sounds really fun to write. Also, feel free to let me know if you have any questions about Americanisms or whether we say particular things.
I swear, the first thing I'm going to do when I make a return to the U.K. (after coronavirus, presumably) is get some quality yogurt. I don't even eat it here, it's so bad.