I feel like it's implying that, Britannia is in a very very weary deep thought process.
I mean, droops can be referred to weary, and pensive means thought, or a deep thought. However, weary could be tired. This is completely an assumption.
well the "as" makes it a simile. its an exaggeration method used to emphasise a certain feeling or action. in this case its used to emphasise a feeling of grief or sadness. Hope this helps!
