oh well i was in france about a decade ago bu will give you my somewhat outdated advice (actually I have been to the UK in the last two years).
as for the CC debate capital one still offers a no annual fee CC with no international fee, so check it out. for her i would suggest a debit card for banks, and at least one CC. Amex cards are not great, my boyfriend has one and when we went to ottawa last year a bunch of places wouldn't take it. as for the chip even 10 years ago some stores had a hard time or didn't want to accept my card without a chip, so again maybe look into that. a debit card with strip and CC with chip may be the way to go.
i used cash a lot when i was in france, i ate lunch out everyday and it was easier to use cash in small cafes and bistros.
i was there a while ago so i used a calling card to call home from the house phone. i didn't think to get a cell at that time and there was really no such think as easy wireless internet either.
as for dressing research it. i was told people dressed up more, but was in a univeristy town with tons of students. i ended up living in the two pairs of jeans i packed, trainers, and flip flops. i had dorky american trainers (white, clunky) and wanted to buy sleek pumas, but they didnt carry my size at the one place i found them. i also wore my black ballet flats a lot, the cobblestones there made it hard to wear heels out.
one thing i did that was a lifesaver was i got a foldable duffle bag at
walmart. i needed one for weekend trips and then when i came home i had lots to bring back and had that to us. i have been in binds on just vacations where i need another bag to bring things home, and you can pay a premium for luggage or duffle bags in some places.
also look into health insurance. i was required to have it but had limited with my own US company so i got this international student card that came with ER insurance. my school accepted that for the trip to meet the guidelines.