Jerusalem is a song about the might of the British empire and how it shall never fall, in "The shoehorn Sonata" Misto uses it to show Sheila's patriotism and love for and belief that Britain shall never fall. It also shows that Sheila believes that England is holly and will be protected by god. When she's singing it in the water after the Giang Bee is destroyed its her attempt to not only stay awake, but also to remind herself that Britain is strong and will not fall, despite what she has just been through. There is some irony in this though, at the end of scene 3 when she is singing this song about Britains might and Singapore, Britains fortress that could not be taken by the japanese, had just fallen.