My observation, based on the 40 years that I have lived in the US, is that the average Briton does not grasp what religion means to so many people here.
If the subject comes up, people might tell you that they are Catholic, Mormon, Jewish, Methodist and so on. But, if they answer with “I am a (Bible-believing) Christian”, they are almost certainly Evangelical Protestants, and very likely Southern Baptists, or the more extreme Pentecostalists, Seventh Day Adventists or one of their offshoots.
The short explanation that I can give as to the significance of these identifications is to imagine that 10% of the British population were following clones of Northern Ireland's Ian Paisley, but a little more aggressive. This is not an accidental comparison, as he was educated at the Fundamentalist Bob Jones University.
For the longer version, we need a little history.
The precursors of the Evangelicals were the Pilgrims who arrived in 1620. They had been thrown out of the liberal Netherlands for being intolerant, and encouraged to leave England, after they tried to tell King James how his church should be run. They came with beads to trade, and guns to steal with, but no skill in construction or farming. Had it not been for a few skilled sailors who landed with them, and the help of some local native tribes, they would have starved, and nearly did. A few generations later they had almost exterminated the local native tribes, and were starting to persecute and kill fellow Christians, like Quakers.
Their descendants were quite happy with the idea of slavery. So much so that the Southern Baptists were formed to support that very issue. Racism and exclusion have been a hallmark of the sect ever since. As late as 1916, there were Church picnics in Southern towns, centred around lynchings of African-Americans (https://www.cvltnation.com/nsfw-american-terrorism-lynching-postcards/).
Just as Voodoo and Santeria took African animism and combined it with Catholicism, the Southern states took Christianity and made it American. Take generous helpings of Genesis, Exodus and Leviticus, plus the nastier writings of Paul, throw in the miracles of the New Testament, but ignore the Beatitudes, and you have some of the flavour of the typical teachings, delivered with the energy of a used-car salesman in a bad hairpiece.
I did not understand the adherents and their views of a hateful God until it was explained to me that all of the vengeful ideas and proscriptions were for other people. Members of these churches believe in the idea of faith over deeds, which is expressed as “Once saved, always saved”. In theory, it is supposed to mean that followers of Jesus will be good thereafter. However, followers will tell you that once you are saved, you are going to Heaven, no matter what you do. Hell is gleefully reserved for everyone else, especially Catholics and Jews. While there are current alliances with both, the former are used to get their way with the government, the latter because Revelation describes a massive genocide of the Jews in Armageddon, and they need to be kept around for that.
In short, the whole belief system was a shock to me, having been raised nominally in the Church of England, complete with required Religious Education and church services 5 days a week.