Some other interpretations:
You could imagine it is about a steel guitar (the biscuit), with the concert hall as the kettle. The 'Sinewed stew of strain' is the strings of the guitar. A stage is the table, the wet land, and the waves around it are the sound. The Rock of ages is the modern form of music about freedom.
The kettle could be this forum, and the song could be the song of creation. The 'Stone aged metal' could be talking about Bible stories. Eternal sighing could be the testimony of the righteous martyrs, whose blood speaks though they die, and wanting to join them (not in a suicidal way).
England could be the Rock of ages surrounded by waves, the place where WWII ended, which hopefully was the last world war. Kettles are popular there as are dry scones. It is an island crowded with people, straining together in foggy soup, very old with a rich history.
It could be a poem about getting drunk. You've got your kettle, your biscuit, your sigh. There's the table with everyone sharing their pains, and the wet land surrounded by waves is everybody getting a buzz.