And if a double-decker bus crashes into usBecause of the structure, grammatically, dying "by your side" is only "a pleasure," a "privilege," and "a heavenly way to die" if the method of that death is a collision with a "double-decker bus" or "ten-ton truck."
To die by your side is such a heavenly way to die
And if a ten-ton truck kills the both of us
To die by your side, well, the pleasure, the privilege is mine
That might be splitting grammatical hairs, but I think it's an important distinction that indicates something about the singer/speaker's outlook: dying is positive only if it occurs in the manner that he's described.
Additionally, until I transcribed the lyrics, I hadn't really noticed how parallel those lines are. "And if a double-decker bus crashes into us" parallels with "And if a ten-ton truck kills the both of us." And "To die by your side is such a heavenly way to die" with "To die by your side, the pleasure, the privilege is mine." But there's even parallelism in the alliterative hyphenated adjectives: "double-decker" and "ten-ton."