Let me get this out of the way first. "Composed Upon Westminster Bridge" is an odd topic for Wordsworth. Why? Because he is a romantic poet. A key to the era of romanticism is nature. Romantic authors heralded the beauty of nature, the perfection of nature, the spiritual power of nature, etc. Nature is awesome. So when Wordsworth writes this poem and talks about how beautiful a man-made city is, it's a definite departure from a standard romantic topic.
That brings me to the first language technique. Exaggeration. "Earth has not anything to show more fair:" Really? The city of London is the MOST beautiful thing you have EVER seen? He is exaggerating of course to help him sell the point that the city in the morning IS beautiful. The hyperbole continues in lines 9-11, too. He says that he has never felt so calm and has never seen a more perfect sunrise.
A simile is contained in lines 4-5. "This City now doth, like a garment, wear