Having been inspired to go and re-read the books...

Holland being British (or born in London, as he puts it) doesn't show up until Fare Play. This was the first of the books that I actually managed to get hold of, so for me Holland was British right from the start. But paying a bit more attention to this aspect on reading "You Have The Right To Reamin Silent", I presume he doesn't have an obvious British accent, or Marian would have been wondering what a Briton was doing in the FBI.