Several references to Shakespeare are scattered throughout the novel (check out "Shout-Outs" for more on this). Both Clarissa and Septimus love Shakespeare, and words from Cymbeline haunt them both: "Fear no more the heat o’ the sun / Nor the furious winter’s rages" (1.15). Love of Shakespeare is part of what compelled Septimus to enlist in the war in the first place; he became so proud of being English that he grew to see England as truly worthy of being defended.
It is maybe stranger that Clarissa loves Shakespeare so much, given that she's not known as a big reader. Still, the lines from Cymbeline – which she (appropriately) reads in a shop window – resonate with her fear of death. Like Septimus’ death at the end, these lines communicate the possibility of not fearing death. Strangely, Clarissa also identifies with Othello’s feelings when she thinks about Sally:
"If it were now to die 'twere now to be most happy." That was her feeling – Othello's feeling, and she felt it, she was convinced, as strongly as Shakespeare meant Othello to feel it, all because she was coming down to dinner in a white frock to meet Sally Seton! (1.16).
Hmmm, Othello doesn't make us think of Sally Seton, but clearly Shakespeare is an important symbol that amps up the Britishness of the whole novel.