@kathleen I really appreciate your thoughts which help in interpretation of the data. I can’t add much more to that really.
This struck me. I imagine the UK can’t be that different from the US, although I haven’t looked at any polls for this. Culturally, there are a lot of crossovers in Christianity, although we may differ regarding some stereotypes of American Evangelicalism, perhaps. I speak fairly generally though. What I can say is that over the last two years, I’ve witnessed Christians become very polarised about how Christians should approach the pandemic, to the point that it most certainly has overpowered grace. It’s been an issue that I’ve witnessed divide members of the body of Christ to the extent that they cannot meet together any more. This breaks my heart. It’s taught me a very big lesson though, that the Gospel is central to everything, and that very likely, God will need Christians who hold to both sides of the argument, for the sake of sharing the Good News with people in different communities. Nevertheless, it can be easy to lose sight of the centrality of the Gospel when such hurts creep into church. I imagine the last couple of years have a lot to answer for regarding this issue of declining Christianity.
I’m reading a book about the Reformation, which I don’t think I’ve mentioned in any of my posts yet 
and it highlighted to me something really important regarding what Paul Miller wrote. The author, Michael Reeves says,
What is perhaps most telling about the reformations in England and Scotland is how very different they were, both from each other, and from the reformations in Wittenberg, Zurich and Geneva. Simply put, a reformation driven more by theology looks quite unlike a reformation driven more by politics. For the kings and queens of England, politics was central to their thinking in a way that just was not the case for Luther, Zwingli and Calvin.
(highlighted words, mine)
Looking at British history through this period shows us that because the church-state relationship was driven first by politics before theology, the division in the country, and death tolls were huge (for those days). I see the same problem today in the West. As long as the church and the state are partnering, and the state drives the culture of Christianity (and the manipulation of it, perhaps), it is opening the door for division, loss of grace, and consequently, a lack of belief because the foundations are not of God, they are of man. I don’t think we can see a clearer depiction of the parable of building on rock or sand than this. Yet, don’t we also see in these sorts of events through history, God break through with a move of His Spirit? Perhaps we can pray for this.