Which aspects of Romantic literature can be perceived in William Blake's poem "London?"

Romanticism is characterized by the glorification of nature, the celebration of the individual, and the emphasis on imagination and emotion. So, with "London" we don't see those themes overtly. But the poem is a scathing critique of the city, tradition, and institutions. This suggests a preference for nature, individuality, and freedom. 

Blake condemns the culture of the city. Since London is the capital, he is condemning English culture in general. Blake believed that people were brainwashed ("mind-forg'd manacles) into accepting this (then) modern, urban way of life. Blake sees the city, government, and the church as oppressive institutions. The Soldier is sent off to fight and possibly die. Blake even critiques marriage when he refers to the "Marriage hearse." People accept that marriage is a necessary path in life and this potentially leads to loveless unions. As a result, some are driven to prostitutes. He is saying that a tradition (marriage in this case) can imprison people and suppress their emotions. 

Blake's critique of tradition and institutions is part of the Romantic themes that we see in other of his works. His critique of tradition and the city suggests alternatives such as individuality, breaking with tradition, and the notion that nature provides an alternative to the urban decay he sees in city life.