The speaker begins with apostrophe, calling out to the 17th-century poet John Milton. This is startling for a few reasons. "London, 1802" is a sonnet composed primarily in iambic pentameter, meaning its lines should have five sets of poetic feet that follow a da DUM rhythm. But this opening line is jarring because it begins with a trochee—DUM da, the inversion of an iamb.
Starting with "Milton!" jerks readers to attention, immediately inverting the unstressed-stressed pattern of iambic pentameter by using a strong stressed-unstressed metrical foot. This trochee ("Milton!") is then followed by a strong caesura in the form of an exclamation mark. This pause only adds to the emphasis placed on Milton's name, as if giving readers a moment to fully absorb the importance of the speaker's address. Overall, this is an abrupt and alarming beginning, one that communicates the speaker's passion—a passion that, in turn, communicates both a deep respect for Milton and a sense of desperation, as if the speaker is frantically eager to communicate with the famous poet.
As the first line continues to unfold, it becomes clear why the speaker wants so badly to invoke Milton. Indeed, the speaker wishes Milton were still alive, noting that England "need[s]" him. This assertion alerts readers to the speaker's dim view of the country, ultimately suggesting that England can't survive without the help of a long-dead poet.
To that end, the speaker believes that England has become a "fen," which is a low, marshy body of water. And, the speaker adds in the beginning of the third line, England is not just a fen, but a fen "of stagnant waters"—a metaphor that depicts England as having lost its energy and momentum ("stagnant" means that something isn't moving).
This is a rather straightforward metaphor that functions as a critique of England's lack of rigor or progress, but it's also worth further examining the speaker's use of the word "fen." A fen is a marshland, and marshlands frequently flood. With this in mind, the speaker subtly implies that the general integrity of England has eroded, much like the muddy banks of a swamp that endures periodic floods.
It's also worth noting that the speaker's depiction of England as a "stagnant" swamp aligns with the opposition many Romantic poets—like Wordsworth—felt toward the Industrial Revolution. Of course, most people think of Industrialization as a period of growth, change, and progress, but the speaker clearly sees it differently, suggesting that England "need[s]" a thoughtful poet like Milton to restore it to its pre-Industrial ways. This implies that the speaker doesn't think the country has made progress, but has stalled out despite its technological advancements and, more importantly, lost something valuable along the way—something Milton could restore to the nation if only he were still alive.