Not much of a spoiler here, so no real warning.

One of the things that caught my attention in the film 1917 was the way that cultural touchstones were shared. Among them, there's a scene at the end where a soldier is synging a hymn, from memory, no songsheets, and everyone is sitting listening. Another is near the beginning where the General Erinmore, sending the Blake and Schofield off, explains his choice with a line of poetry.

This of course brought to mind the Copy-Book Headings that Kipling wrote about.

More importantly though, there's a point where Schofield, in a meeting with someone, turns to poetry, also recited from memory:

They went to sea in a Sieve, they did,
  In a Sieve they went to sea:
In spite of all their friends could say,
On a winter’s morn, on a stormy day,
  In a Sieve they went to sea!
And when the Sieve turned round and round,
And every one cried, ‘You’ll all be drowned!’
They called aloud, ‘Our Sieve ain’t big,
But we don’t care a button! we don’t care a fig!
  In a Sieve we’ll go to sea!’
    Far and few, far and few,
      Are the lands where the Jumblies live;
    Their heads are green, and their hands are blue,
      And they went to sea in a Sieve.

The full poem is here.

While Firth's Erinmore was an officer, and could be more reasonably expected to have poetry on tap at the drop of a hat, Schofield wasn't.

This was a culture that even the common man would know pieces of poetry, and officers and men might recite them when it fit the need.