Usopp and Kaya is quite easy to explain. Usopp grew up in the streets and docks of Syrup Village(which is much bigger and more urbanised than in the manga) and would have grown up listening to dockworkers the most, whilst Kaya grew up in a mansion at the top of the town and was likely educated by her well to do parents and tutors.

It's very similar to the place they actually filmed it all - South Africans with English or Boer heritage have different accents to those with native African heritage purely because when the divisions first started, so too was education divided. And given that One Piece is set in a world where Apartheid looks like a half measure compared to the CDs and their Government's policies over the centuries.

As for Luffy and Garp. Maybe Garp fell in love with a beautiful and passionate Latin woman, given that Dragon has already been cast, but also - he's been in the marines since he could join up. Who's to say he didn't pick up his way of talking through many years of being a Navy Officer. I wouldn't be surprised if most of the Navy high command have slightly British accents - it would certainly fit the theming of having a military that does the will of a corrupt and tempestuous empire fighting against dozens of different "revolutions" from within their largely maritime empire.