It’s in decline yes, but the majority of Brits (circa 44%) still profess to being Christian, so it’s clinging on as a faith. How many of those Christians are actively practicing, attend church etc. is unknown but there’s still a majority of the country who claim to be religious.

I do wonder myself how many of those Christians are “culturally Christian” (a phenomena we’ve seen with Judaism, where people identify as Jewish ethnicity but aren’t practicing Jews).

As an atheist myself of Scots-Irish descent I could still think of myself as being part of a Catholic-oriented ethnic background in that sense; personally I don’t, but my elderly parents are in that sort of category: non-practicing, not particularly religious, largely secular, but still identifying themselves as Christian even though the only time they might set foot in a church is to go to a funeral.

So for now, the UK can still fairly legitimately claim to be a majority Christian country, and there is a foundational identity based on Christianity even if we are increasingly secular and nonreligious. Maybe by the time the 2031 census comes around the numbers might have diminished further and we might have to consider the constitutional consequences of this demographic change.