Background: I have a character, 19-year old upper-middle class girl, born in the Netherlands originally speaking Dutch but moved when she was 8 with her family to London in a roughly-modern (2000ish Earth) setting. This being said, she learned English from an older, stuffy tutor and therefore says things that are slightly outdated/strange (an example being 'I shall do that for you' among others. I needed to look up the grammar rules for that one). That, and it's her personal preference to speak that way because to her it sounds more 'proper.' Edit: Forgot to add that she wouldn't have had much exposure to 'outside' people and probably didn't go to public school; she was a child actress and spent much of her time with either her family or tutor.
I had a few questions about certain terms that seem to be UK specific and have yielded confusing results when referencing it online or talking to other people.
Words and phrases I'd like some help with if possible:
While vs. Whilst = I've combed quite a few English grammar forums and it seems like no one can agree 100% on which one's the right one. Predominantly, 'while' seems to be the more modern choice, but there are also people saying that 'whilst' can be used in certain contexts and conversations (formal writing, if you generally prefer using it over 'while', etc.). I'm not sure which one a character in the above situation would lean towards if either one, or if there are rules to use it. Does it all chalk up to personal preference, or is there a culture thing I'm missing?
Meant vs supposed to = I've seen this a couple of times in the context of 'I was meant to be in my friend's wedding but overslept and missed it.' My friend from Newcastle says that she'd never use 'meant' in this fashion, but since I've seen it used, I wondered if it was a regionalism (her sister said she's used one as often as the other in the same context). Is this another personal preference issue?
Also, if anyone has any reliable sites for modern-to-slightly outdated British-isms and ways British English differs subtly from American English, links would be appreciated. I'm as a babe in the woods here. Though I did use Google and some suspect-looking dictionaries before, I'm looking for some confirmation and debunking if something is outright wrong. Thanks to anyone who can help in advance!
Hijo de edit: I've now gotten a lot of helpful suggestions, thanks everyone!