The poem makes clear that London is not entirely responsible for its beauty in the morning. A number of factors, including the unusual absence of any fog and the way the light strikes the ships and buildings, combine to make a perfect scene. Because the speaker knows that such a combination does not happen very often, he thinks that a person would be foolish just to pass by, assuming there will always be other chances to see such beauty. The speaker believes you have to take advantage of such opportunities when you have them.
The poem expresses the speaker's desire to stop time, to prevent the city from ever "waking up."
The image of a beautiful garment implies that the city is like a blank canvas that nature adorns, rather than something possessing beauty on its own.