The very first word of the poem is a moment of apostrophe, as the speaker immediately calls out Milton's name. The title of the poem clarifies that it takes place in 1802, but John Milton died in 1674, meaning that he couldn't possibly respond to the speaker's address. In this regard, the speaker treats Milton as a muse, a person the speaker would like to summon for both creative and spiritual guidance.
Milton is an especially appropriate muse, since his magnum opus, Paradise Lost, opens with an invocation of a heavenly muse, whom Milton hopes will help him tell the story of the "Fall of Man." Apostrophe is a very common poetic device in epic poetry, as speakers often call upon divine figures to help them navigate their way through the rest of the poem.
Of course, "London, 1802" is a sonnet, not an epic poem, but it's no mistake that it uses apostrophe to mimic the beginning of an epic poem. After all, the speaker wants society to model itself after John Milton, so it makes sense that the poem emulates Milton's poetic style. By addressing Milton in the same way that Milton addresses a heavenly muse in Paradise Lost, the speaker gets that much closer to living and behaving like the famous poet himself.
After calling out, "Milton!", the speaker continues throughout the rest of the poem to address the famous poet, ultimately framing him as both a muse and a savior of sorts, asking him to "raise up" and "return" from the dead in order to restore British society to its former greatness.