If a British soldier began serving in 1914 in World War One as an infrantyman, what were the odds he would survive to the end?

I'm not sure about the odds exactly, but I am covering this topic in a Global History class I'm currently teaching. There were several major factors that soldiers faced in World War I that they hadn't faced in previous wars. The first were the advancements in modern warfare. Machine guns were much more powerful and deadly than its inventors and supporters initially anticipated. You went from wars like The Civil War (not British, I know, but provides some context) where soldiers could shoot 3 aimed shots in a minute, to guns that could fire hundreds of rounds in the same time. So the odds of getting hit were higher, especially in No Man's Land where there wasn't much to hide behind if you weren't in a trench. You were particularly screwed if you fought on the eastern front near Russia where you not only had to deal with bullets, but also the frigid temperatures that killed hundreds of soldiers daily.

The other advances included chemical warfare and the advent of air warfare. Chemical warfare wreaked havoc on WWI battlefields because exposure to it, even if you had a gas mask, left you with lifelong health problems, if it didn't kill you first. My grandfather was left with a breathing problems for almost fifty years after the war ended right up to the day he died because of his exposure to gas. Then there was air combat. This was the first all out war where airplanes were used in a combat role, meaning that you weren't just exposed to gunfire on the ground, but also from the air.

Soldiers on both sides weren't quite prepared for this onslaught of warfare because its leaders' couldn't quite anticipate it because they hadn't seen it in their own experiences before. For many American soldiers, their experience in war stemmed from the Spanish American war, which lasted only a few months.

While this is just a general response, I hope it gives you some insight.