If it's England then wouldn't it be "Father Christmas"? I'm sure that my history of Santa Claus in different countries isn't all that great, but if you're aiming for some historical accuracy it might be important to know. I'm pretty sure that "Santa Claus," as a name and much of his characteristics and manners, was an American invention during the Victorian time period. It probably influenced 1940s England, but I think it might have still been fairly resent.
All of this is of course too much information for an audience of 8-12, but it might be important as an author to know, especially if you want him to wear red or green, for example. And if you have if the fairies in your book know the "real" Father Christmas and you are just saying that Santa was made up, then it has a different feel than if you are saying that the magic of Christmas is an invention of Coco Cola in a successful marketing attempt to increase winter sales of a cold beverage (or Macy's, which I personally like even more, but wasn't until 1947).