I read this story in my teens, either as part of secondary school (UK), or from the local library in the late 1990s or early 2000s.
There has been some sort of apocalypse caused by I think a flu like virus that has killed a large number of the population. The story focuses on two girls who are travelling through a city (possibly London).
One girl has been affected mentally by the ordeal and has problems with the situation.
The only real scene I can remember is that the mentally affected girl is wearing a wedding dress and in the middle of the street takes it off and the other girl notices someone in a window twitching the curtains and she believes that they may be undertaking a sexual act, while the girl is undressing.