Christ people have a stick up their arse on here, particularly when someone from England mentions something which is unusual in England, or at least the region they grew up in, but is commonplace in Scotland, Wales or Northern Ireland. Sometimes people may be being goady, other times it could be completely innocent, and yet is instantly jumped on with "England isn't the only part of the UK!".
He's Father Christmas to me, never had a nursery and I have a regional accent. It sounds "right" to me, because its what I've known since childhood. Santa on the other hand sounds somehow "wrong". Obviously this will be reversed depending on what you called him in childhood. It doesn't really matter, the same as Mum, Mam and Mom.
I think the question can be asked though, without it being seen as sinister, when a cultural shift occurs that seems to be changing a regions language occurs and where the influence has come from.
People get defensive because they feel it is attacking their version of normal, when in reality its questioning why another areas "normal" seems to be changing.
As for pointing the finger at it being an Americanism, thats simply because it seems the most likely cause. It's unlikely kids in Northants or Buckinghamshire are calling him Santa because people in Scotland do. Its more than likely because of all the movies and songs which have come from America and that is what they are exposed to.
If the kids in your area started calling him Ded Moroz or saying Boldog Karacsonyt instead of Happy Christmas, yet there were no large Russian or Hungarian influences or communities, would it be wrong then to question why this usage has become more commonplace in places it previously wasn't?