In this poem, the speaker is longing for what he he considers to be England's Golden Age, the Enlightenment. This is the theme of the poem.
The Enlightenment is widely defined as the years between about 1650 and 1800. The poem's title, then, "London, 1802," is set just after the end of this period. The Enlightenment was a time when reason and science were favored over superstition and religion. It was a time when many people believed that everything in the universe could be known and the arts flourished as a result of this new freedom of expression.
The Milton to whom the speaker emphatically invokes in the first line is John Milton, the author of Paradise Lost, written during the Enlightenment and was, both then and now, considered to be one of the world's finest literary texts.
The speaker bemoans what has happend to his homeland. He uses the figurative language of a "fen" (that is, a swamp or bog) to describe the state of the country. All, in his opinion, has deterioratied: altar, sword, and pen (that is, religion, the military, and the arts).
In the following lines, he invokes, using figurative language, the previous granduer of the royalty of England, saying, "Fireside, the heroic wealth of hall and bower / have forfeited their ancient English dower / Of inward happiness." A "hall" references the festival halls of palaces, a "bower" is a garden room of the castle; a "dower" is a dowery, that is, the money of a deceased male's real estate given to a widow after her death. So, what the language brings to mind here is a castle, that is, England, in decay. This decay has been brought about by the residents (Englishmen) themselves for turning their backs on the previous granduer and ideas of the former age.
The speaker pleads with his mentor, Milton, to bring them back to their former state of greatness. He describes the older poet as being like a "star" and his voice was like the "sea." The poet gave light and guided; his voice is everlasting and powerful.
More figurative language concludes this brief poem. For the speaker, Milton is akin to God himself. He is "pure" and "majestic" and like Jesus, he came to Earth in "cheerful godliness" to assume to the "lowliest of duties," that is the betterment of mankind.