Question

What does the line "Britannia droops the pensive head" imply?

Answer 1

Britannia is the female symbol of the British Empire. She (and the British Empire) are represented here as ancient and tired and responsible for many deaths in her thirst for power and race of imperialism.  

Answer 2

The line implies gloom in Britain as a consequence of impending defeat in the war and shame for the misdeeds committed before the war.