Symbolism, Imagery, Allegory
Get out the SPF 30, because the sun shines bright in this play. Imogen might be talking about her own rough situation when she talks about the sun to Pisanio, but what she says sums up a larger idea in the play. She asks: "Where then? / Hath Britain all the sun that shines?" (3.4.157-158). Translation: Britain is basking in the sun. It's the best and has all the good stuff in life. Why go anywhere else?
Later, Imogen's brother explains that he will fight for Britain, and he uses this imagery to talk about his decision, as well. Arviragus announces: "I am ashamed / to look upon the holy sun, to have / the benefit of his blest beams, remaining / so long a poor unknown" (4.4.48-51). Arviragus sees how sunny and bright Britain is, and he wants it to stay that way. Of course, by the end of the play, thanks to him (and his brother and brother-in-law), it will.
The Soothsayer interprets the oracle to mean that Rome "lessened herself, and in the beams o' th' sun / so vanished"(5.5.572-573). It looks like Rome forgot to wear its sunglasses when it stared down the sun. Because Britain is represented with the image of the sun, the suggestion is that it will keep shining long after the play is done.