One figure of speech in the poem is personfication, where inanimate objects are given human-like traits. Wordsworth, right from the beginning, calls England "she," which makes her seem like a person, not a country. Then, he personifies the "altar, sword, and pen," by saying that all three of these have "forfeited" their capability for bringing internal happiness to people. Only people can forfeit things; it implies intent, forethought, and an element of surrendering your will. So, to have a pen, altar and sword forfeiting something is using personification. The last example of personfication is when Wordsworth calls Milton's heart a her, and says that "thy heart the lowliest duties on herself did lay," personifying the heart, saying it laid duties upon itself.
Another figure of speech is a simile, where you compare two things using "like" or "as". Wordsworth says Milton's "soul was like a Star" and had a "voice whose sound was like the sea". He uses these similes to more profoundly describe how he longs for Milton's beautiful poetry and stories to come back. Wordsworth mourns the loss of romance and beauty in England, and implores Milton to return and infuse those glittering qualities back into society.
Those are just a couple figures of speech; I hope that they help to get you started. Good luck!