Ooh language debate about correct pronunciation - Zee or Zed are both acceptable.
As for Americanisms, these are in fact Britishisms. Don't believe me, then ponder where their language came from, where their people came from (mostly) The same is said for spelling, just ask Shakespear. That very British author and play-write wrote his surname in several different ways.
As for the show, well the TARDIS translation circuit decides what you will actually hear, so I don't know why anyone would get their knickers/panties/pants in a bunch/twist/knot