The overall message of the song is one in which the animals sing of a world without human beings. The message of the song is one where there is triumph of the animals over the exploitation of the humans. Old Major sings this to the animals to emphasize his overall vision of a world without human interference and intervention in the animal world. Along with the commandments of Animalism, the song is an element that brings out a sense of unity and community in the animals. It helps to coalesce them as a group against the humans such as Jones who abuse the animals. The song is an example of the animals' cause being something that is more than simply a vain wish or idea, but rather an actual set of beliefs in which the animals are able to envision of a world of what should be as opposed to a world of what is. It is here where I think that the animals are able to start the process of reclaiming and identifying a voice, something that they are denied in the world of the humans. It is this message that is so very important in the song and in the course of the novel.
In Animal Farm, the message of "Beasts of England" is that the animals must rebel against their human masters in order be free and equal. To emphasize this message, the song portrays humans as tyrannical ("Tyrant Man") and portrays life under human rule as miserable and oppressive. This is shown most clearly in the third verse of the song, in which animals are physically attacked by Man with "cruel whips" and are pictured with rings in their noses and harnesses on their backs.
Moreover, to convey this message, "Beasts of England" offers a glimpse of what life will be like when Man has gone. There are ample "riches," like wheat and barley, for example, the air is "sweeter" and the fields shine brightly.
Towards the end of the song, the animals are encouraged to overthrow Man to make this idyllic dream a reality.
All must toil for freedom's sake.
That the animals learn this song by heart shows that they are desperate for change, and "Beasts of England" quickly becomes their adopted anthem.