It is declining in the UK and the West in general because
Christianity was never a Western religion in the first place, and it doesn't particularly meld well with the hyper-individualism that has been traditional to the West for the past few centuries (coming out of the Enlightenment and all that), nor with the rapidly emerging political tribalism and aggressive secularization of Western societies.
Not to mention that there is absolutely nothing in Western Christianity of any kind that cannot be found in roughly equal perhaps even greater measure in secular environments, once you've done away with the explicitly religious metaphysical claims, as many forms of Western Christianity have in favor of this or that flavor of 'social gospel'.
So you have a bunch of people who don't necessarily believe in anything in particular (since the metaphysical claims are either weakened past the point of being taken seriously by any large number of people, or just jettisoned all together because what
really matters is something else), in an environment that does not require of them any particular belief (and even actively discourages it, unless that belief is in a non-Christian religion, in which case it's beautiful and holy and can't we all learn so much from it), at the same time being invaded by the dregs of third world Islamic hellholes who
very much believe in the very religion and accompanying political system which made their original home countries into such shining beacons of freedom and liberty which surely everyone wants to live in.
Yeah...it's a real mystery you've got there.