If I remember correctly, in the 2011 census, something like 52% of people in the UK claimed to be Christian, while only 6% went to church with any regularity.
Now, there will of course be people who, due to living very far away from churches or being too ill to get out or working funny hours will be unable to attend church. And there will be people who pray at home/ think churches are idolatry etc.
But I think it's quite safe to say that the remaining 46% mostly belong to the category of "I don't believe in it/never gave it much thought but I quite like Christmas". I mean, if I believed in an all powerful God capable of damning me for eternity, I'd bother turning up at least occasionally.
I think, often, the religiosity of the past is overstated. A lot of people seem to have an idea that, until 1960, virtually everyone went to church and believed in God. I've certainly met people who believe this. In reality, while people declaring themselves outright Atheists probably wasn't common (save for certain academic circles), plenty of people weren't ardent believers, even if it was mostly through apathy than anything else.
However, even if individuals didn't believe in God, the official line was still that Britain was a Christian country and the church retained their power and influence respectively. But the church is increasingly incompatible with people's own beliefs. Over generations, you've had more and more people break away from it as they've been offered a more liberal, appealing alternative.
I do not, myself, believe in God. My family's background could be described as belonging to that unofficial but wide reaching branch of Catholicism whose primary aim seems to be that you never, ever, ever turn to Protestantism. Anything beyond that was a bit half arsed.
I did, as an adolescent, spend quite a lot of time with more traditional Catholics who, though some of what they came out with was questionable to say the least, were very kind to me and provided me with a stable environment which I hadn't had with my parents.
So my own experience of Catholicism has probably been more positive than most. (Probably also helped by the fact that I went to school with a lot of Evangelists, Pentecostals and Calvinists, who made Catholicism look really quite benign). Despite the atrocities committed by the Catholic church, I have a certain, if vague, loyalty towards it. If I were to go to church, I'll go to a Catholic one.
So I know there are good religious people. I know that there are incredibly intelligent religious people. But my perception of them as generally good and intelligent is challenged when I see them tying themselves in knots to explain why an omni benevolent, omnipotent God lets children die of Leukemia before, almost without fail, saying that none of us can truly understand God.
Because a lot of it is not just outdated, it's illogical. And saying "But you can't prove that God doesn't exist" is a piss poor argument. I can't infallibly prove that unicorns don't exist. They might just be really hard to spot. Or, better yet, they might exist in a way in which no human can truly understand, have incredible powers which they use in ways we can't hope to understand, and love us in ways we can only begin to imagine, though it does seem to involve quite a lot of terminally ill children. But, hey ho, who are we to question the unicorns.