If I should die, think only this of me, That there's some corner of a foreign field That is forever England (1-3)
The speaker's patriotism is evident in the fact that, when thinking about his own death, the "only" thing he really thinks about is how it will benefit his country. England will acquire a "corner of a foreign field." (Of course, he might be convincing himself that his death will be a worthy sacrifice, too.)
A dust whom England bore, shaped, made aware, Gave, once, her flowers to love, her ways to roam (5-6)
The speaker's love for his country is reflected in the way he perceives her as a mother, a parental figure that "bore" him and "shaped" him. He is willing to die for his country because she raised him and made him who he is.
In hearts at peace, under an English heaven (14)
The speaker's Englishness permeates all aspects of his life. He's so pro-England that he even imagines heaven as just another England, for criminey's sake. It is an "English heaven" complete with all that "thoughts by England given" (11).