When I get round to discussing it I'm going to use proper statistics because there is available data to show that overall American culture is more religious than British.

I'd argue that's your misreading of English culture. I'm an atheist and an atheist of Jewish extraction and I don't see either Easter or Christmas as being Christian. They were once but aren't now.

And you asking people to think of that as being Christian practice and therefore disrespectful of you is almost doomed to failure because it requires them to change their understanding of a non religious cultural practice to being religiously significant and to accept that you have beliefs that are important to you which is again profoundly unEnglish unless it's about football. Belief is private and should not be discussed.

I'd respect your wishes because it's polite to do so but ... I still wouldn't see a Christmas card as implying any religious belief.

otoh I have the very devil of a time getting some American flistees whether Christian or pagan to accept that I'd be awfully grateful if they wouldn't hold me in their prayers and in particular not offer me their blessings because not only am I an atheist, that assumes a hierarchical relationship with the nonexistent deity with them higher than me.

I find church schools a worry. A good education shouldn't depend on belief.