Been thinking about this and I think I have to say OP YABU because whatever you call him we're talking about the same American symbol really...OK so Father Christmas might be more English with pagan/christian crossover background and Santa might be used by the Scots and Irish because they don't have the tradition of the figure of the old man of Christmas or whatever.
I don't know, nothing to do with English parochialism here (my family are Irish anyway) but I still think all the evidence points to S word coming to us from America rather than direct from the Dutch - I asked my philologist friend and that's what he told me, even in the case of Scotland (who correct me if I'm wrong because I could easily be
, historically never went in for big Christmas festivities due to the influence of the Kirk who at its dour strictest, viewed Christmas as an idolatrous celebration not to be observed - hence why the Scots go in Hogmanay in a big way, they could let rip then instead).
Google reveals that "The transformation of Saint Nicholas to Santa Claus happened largely in America ? with inspiration from the Dutch. In the early days of Dutch New York, Sinterklaas became known among the English-speaking as "Santa Claus" (or "Saint Nick"). In 1809 Washington Irving, a member of the New York Historical Society (which promoted a Dutch Saint Nicholas as its patron saint), created a tale of a chubby, pipe-smoking little Saint Nicholas who road a magic horse through the air visiting all houses in New York. The elfish figure was small enough to climb down chimneys with gifts for the good children and switches for the bad ones.
The 1823 poem "The Night Before Christmas" ("A Visit from Saint Nicholas", reputedly by Clement Moore) replaced the horse with a sleigh drawn by eight flying reindeer. (Moore may have been inspired by the Finnish legend of Old Man Winter, who drove reindeer down from the mountain, bringing the snow.) Following Irving's example, Moore's St. Nick was more an elf than a bishop. Unlike the earlier St. Nicks, this one brought no birch switches, only presents. And it was Moore who established that St. Nick brings presents on the night before Christmas rather than on Saint Nicholas Day or any other time."
So I think at the end of the day who cares, call it what you like and is traditional from your region (bearing in mind that Christmas as we tend to celebrate it now borrows heavily from America anyway - turkey anyone? cranberries?) I tend to say Father Christmas but actually we mix it up interchangeably with Santa, Santa Claus etc.
Getting so riled about it really just isn't in the spirit of Christmas 