Here we go quick history lesson for you OP.
Lesson 1: We know that the British Isles had pockets of Christianity by the fifth century. We know this because a chap called Gildas left us some notes about it. What we know through Gildas is there were two forms of Christianity Catholic (brought over with some Roman dudes) and Celtic (it's unclear where and when Celtic Christianity arose but it was the prevailing Christian faith).
There are some myths/Legends that Jesus did indeed come to these lands (possibly to Glastonbury) however this has never been proved, what Historians do know is there is a period of time that Jesus ' goes quiet' (for want of a better word) and it has been hypothesised that he was travelling and spreading his word).
Lesson 2:
Christianity took hold and was the predominant faith in the British Isles from the sixth centenary through the early twentieth century, it is still a main faith in the Uk, despite declining numbers of bums on pews it's still very much part of our society (eg many of our laws are built upon Christian principles).
Lesson 3: (now I am having to rely on my history lessons from over 30 yrs ago so my dates might be slightly out)
Remembrance Sunday- WW1 officially ended on the 11 th hour on the 11th day of the 11th month. The first Remembrance Day (or armistice day as it was originally known) took place on 11.11.1919. To commemorate those that died in the 1914-18 war.
Lesson 4 back to Christianity.
The most important date for Christians is Easter. Why? Put simply because Jesus rose from the dead (this is the single most important aspect of Christianity and one I'll come back to)
Christianity was one of the first religion and continues to be one of the main religions believing in life after death FOR EVERYONE!
Lesson 5: think back to a time before science, before we had an understanding of illness, when mortality was high and life was cheap, when we had no (or very limited) understanding of the human brain, think how comforting it was to know, some dude had walked among his people saying "Wotcha, I'm still here, body gone spirit lives on...follow me and you can have this too" I mean just think about it, what's not to like? suddenly death doesn't hold its fear for the dying, for the bereaved it gives comfort, how amazing to think you'll see your loved one again.
Lesson 6: religion on Fire! So now the embers of Christianity slowly smoulder, and the movement gets more momentum. The Roman Empire is gradually disintegrating, and the power of empire is losing its grip, with various tribes slowly taking back their lands (no one likes to be oppressed....something dictators can't seem to understand) and gathering armies. These are troubled times for all, what do humans fundamentally do in troubled times? Call out for help, this help is answered in the form of a new fangled religion (only a few centuries old). So now Christianity is growing in strength and predominance. By 800AD Christianity is the main religion in Europe (including the UK) and the churches power continues to grow. Pretty much from the reign of Charles the Great, Church and State are now so intertwined that one can no longer survive without the other.
Lesson 7 historical stuff: so apart from Cromwell the Uk has always had monarchy and church ruling together (not always harmoniously King John was excommunicated a good few times for not towing the party line as was Heners 8th who got so fed up he broke away from the Catholic Church and set up his own: the Church of England). Many of our European cousins had more impressive rebellions (Looking at you France!) where State and Religion separated. This never happened in the Uk.
Lesson 8: back to 1919. So the war, which was only meant to last a few weeks, saw more young men, than in any other time, die. Some of these men were barely into manhood. It's was an atrocity on a scale never seen before. Thousands left to Mourn and Christianity was there to give hope, bring peace and allow people to come together in their grief and remember those who died to allow those who were left behind to have their freedom. As the King (1919) and our current king were and are both head of state and Church of England, it should not come as any surprise that when the head of state is present then of course so is the church.
I have written this in the most basic way possible, there is a huge amount I have left out, it's a hugely complex area and I'm neither historian nor theologian, but I hope that goes some way to answer your question of why are so many state events Intertwined with Christianity (at least I think that's what you are asking? You discussion points seem to shift around a bit).