Just to elaborate a bit, as I am the poster who said that, to me, people who say “Fathah Christmas” are the ones who have nurseries and eat “suppah”.
It was a light-hearted comment, in no way intended to denigrate people who meet that description. What I was trying to convey was that saying FC not Santa was very removed from my own experience growing up in a lower middle class home in Scotland. (Protestant, by the way- I don’t buy the Catholic/Protestant distinction suggested up thread).
What I didn’t mention in my lighthearted comment was that as I child I devoured every Enid Blyton book going, watched Mary Poppins and the Sound of Music hundreds of times and worshipped Julie Andrews (still do!). The “nursery and suppah” culture where children talked about “Fathah Christmas” was not at all negative to me, just very different. It occupied a sort of fantasy land from books and TV. (As were Grange Hill and Corrie- it wasn’t only the posh English who seemed foreign!)
As I think I mentioned in another post, my husband was brought up to say Father Christmas and my MIL (West Country born and bred but upper middle class) definitely found my “Santa” to be a marker of lack of refinement. What is funny is that MIL and my own late Mum are quite similar in their world views, but “Santa” carried no such connotation for my own mother. (She had plenty of other class markers to look down upon instead, for example her abhorrence of the Scottish habit of saying “juice” to mean all cold drinks including fizzy ones). So I could sort of see where she was coming from when she told my DS that “we say Fathah Christmas”. I corrected it to “Granny calls Santa Father Christmas” and we are all happy now (DH couldn’t care less and it’s actually quite sweet to see him correct himself to Santa when he starts to say FC out of habit).
Finally, going back to the OP, I do actually agree with the notion that many kids in England have picked up Santa because of American culture (not recently though, it’s been going on for decades!). What rankled for so many people was OP’s seeming complete ignorance of the fact that “Santa” was the name used almost exclusively in huge areas of the U.K., dating back to from before American culture really took hold.
If it had just said “I know Santa is traditional elsewhere in the U.K. but where DH and I grew up most kids said “Father Christmas” that seems to be dying out now I think that more and more people are saying “Santa”. I wonder why that is- could it be American culture?” then things might not have kicked off so spectacularly.