Why didn't Costner even bother to speak the same English as everyone?

If you listen to the rest of the (British) cast, everyone uses clear and obvious British slang and words typical of England but not of USA.

So having Costner not only with a clear California accent but also lacking even one single British slang ("bloody", "bollocks", "mate", etc), it makes him sound like he's from another country entirely (he's supposed to be British also), even from another movie.

What was the point?
- Was Costner too much of a prima donna back then to even try to blend in and make an effort?
- Was is to make him sound distinctive? He's supposed to be from the same place everyone else is!

I don't care for the right/proper accent/speech/language of the time (nobody would be able to understand them), but having people from the same place (in the movie) sound so clearly different makes no sense at all!!