Alliteration, like assonance and consonance, helps bring the poem's many images to life on the page, and intensifies the different moods in each stanza.
The first two stanzas describe the immigrants' perilous crossing from the European mainland to Dover in a small boat. Here, alliteration conjures an atmosphere of threat and potential danger, as well as the terrible weather.
For instance, listen to all the /c/ alliteration and /s/ sibilance in lines 8-9:
camouflage past the vast crumble of scummed
cliffs, scramming on mulch as thunder unbladders
All these harsh /c/ sounds work with consonance (e.g. "scummed" in line 8) to make this whole section terrifying: an onslaught of hard, rocky sounds mirrors the intensity of landing on a pebbly beach in a gale. And the sibilant /s/ sounds evoke the stormy hiss of wind and waves.
Later on, alliterative sounds draw attention to important moments in the poem. Take a look at these lines from the third stanza, for instance:
burdened, ennobled, poling sparks across pylon and pylon.
Here, the speaker pictures the immigrants as a kind of metaphorical "electricity" keeping the country running through their unseen work. The strong /p/ alliteration here emphasizes this idea, connecting one word to another just like electrical wires linking "pylons."
Alliteration also plays a thematic role in this poem. Daljit Nagra has said that he wanted the strong alliteration and consonance here to recall the punchy, repetitive sounds of Anglo-Saxon verse. And Anglo-Saxons, of course, were some of the earliest immigrants to the British isles!