There's quite a bit of alliteration in "Football at Slack," which adds music, rhythm, and moments of emphasis to the poem.
For example, the first three stanzas are filled with bouncy /b/ sounds (primarily as alliteration but also as consonance) that evoke buoyant movements of the "blown ball" and the men running after it. Take a look at lines 1-3, for example:
Between plunging valleys, on a bareback of hill
Men in bunting colours
Bounced, and their blown ball bounced.
The alliteration here is unmissable, all those bold /b/s calling readers' attention to the men's—and the ball's—energetic movements. These alliterative /b/ sounds pop up again and again in the next seven lines, in "blown ball," "ball blew," and the tongue-twistery "blown ball blew back." The language feels bright and playful.
In the fourth stanza, however, this /b/ alliteration gets replaced with a variety of other sounds as the storm interrupts the men's game. Take the husky /h/ alliteration in lines 11-12, for instance, which evokes the heavy, huffing winds that tear through the landscape:
Winds from fiery holes in heaven
Piled the hills darkening around them
The next few lines feature throaty /gl/ alliteration and tight-lipped /m/ alliteration, intensifying the poem's language in response to the growing power of the storm:
[...] The glare light
Mixed its mad oils and threw glooms.
Notice, too, the /oo/ assonance in "threw glooms," which also adds yet more intensity and emphasis to this image of darkness being violently tossed across the landscape like a splatter of paint.
Thudding assonance and alliteration combine again towards the end of the poem, evoking the "depth" of the storm:
[...] unthinkable
Under the depth of Atlantic depression—
Finally, the shared sounds in "goalie" and "golden" and "horizontal" and "holocaust" create a kind of bridge between the last two stanzas of the poem. Readers can envision the football player temporarily suspended in midair, the ferocious sun almost timidly peeking out from behind a cloud to behold him.