Alliteration is a subtle but important presence in "Those Winter Sundays." It's first used in lines 2 and 3, with the harsh /k/ sounds in "clothes," "cold," and "cracking." This sound is even more prevalent in these lines when considering consonance more generally; overall, it's a biting sound that suggests the harshness of the crackling, aching cold.
Later, in line 4, the two /w/ sounds in "weekday weather" create a sense of routine drudgery, conveying the way that the speaker's father has to work hard every day of the week. The next example is in line 5 with "banked fires blaze." The alliteration has a bright, bouncy sound that adds emphasis to the phrase and suggests the sheer strength of this blazing fire.
Once the fire is lit, the poem uses warmer sounding consonants. This is most prominent in the alliteration of line 7, with three /w/ sounds one after another:
When the rooms were warm
The /w/ sound here infuses the line itself with this sense of warmth.
The poem has one further instance of alliteration in the final line (quoted with the preceding line for context):
What did I know, what did I know
of love’s austere and lonely offices?
The two /l/ sounds here conceptually link "love" with being "lonely." This is an important moment because it's part of the way that the speaker, now thinking about his father from the vantage point of adulthood (as opposed to the "indifferent" view of childhood), reconsiders the way his father showed his love. In other words, his father's love was expressed in the way that he carried out his duties and responsibilities—and lack of appreciation at the time made the father into a fairly solitary figure.