Heroine

I agree on the Byron quotation and the pre-Raphaelite stuff but I guess Brett is playing with the different layers of meaning one can read with both those references and his own context.

I think the song is about the discovery of sexual attraction and the idealisation of women, which is constructed from the references you already mentioned, Hollywood stars like Marylin Monroe but also prostitutes, who might be the actual women to whom the boy is attracted to as he lives in a slum.

The most interesting thing about thins song is how all those references work together, going from the innocent attraction described in Byron's poem to the unbearable longing caused by desire in pre-Rafaelite paintings to the tragic figure of Marylin Monroe and, finally to the prostitutes and the unavoidable reference to heroine. One meaning doesn't exclude the other and actually that's one of the great things about this song, it is about a young man who is so attracted to a woman that she is like heroine to him and at the same time about an addict whose desire for the drug is as strong as that of a teenager for an idealized woman.