In Animal Farm, what does the song "The Beasts of England" symbolize?

The song is both a battle cry for the rebellion on Manor Farm and an anthem that helps the animals keep the spirit of the rebellion alive in their hearts.  After Manor Farm becomes Animal Farm, the feeling among the animals is that things will be better now that they are ruling themselves.  They are no longer under the rule of the humans who has taken us for granted.  The beasts of England are the humans who have abused the animals.

When the animals successfully defend the farm against the humans in the Battle of Cowshed, the song is even more important to the animals. 

"News of the Rebellion at Animal Farmbegins to spread, and animals across the countryside are singing "Beasts of England." The neighboring farmers, led by Mr. Pilkington of Foxwood and Mr. Frederick of Pinchfield Farm, attempt to retake Animal Farm by force."

Also 

The “Beast of England song” song is important because it is what unites the animals in the beginning of the book it is a symbol of change for the animals. “Bright will shine the fields of England, purer shall its waters be, sweeter yet shall blow breezes on the day that sets us free.”  The animals are singing a song of freedom, the animals are expecting their desire to leave the dictatorship of Mr. Jones and start a democracy. The fact that the animals sang the song after the battle of cowshed is an example of how it was a symbol of freedom because they were finally free from human rule the animals could now rule themselves.

Also, when the animals are no longer aloud to sing the song it shows that the animals are slipping back into the type of leadership that was in place when Mr. Jones was in charge of the farm. It was also one of the last straws that the animals took before over throwing the pigs. The song being taken away symbolizes that the animal’s freedoms are also being taken away, the freedom that they fought so hard to take from Mr. Jones and now the animals must now fight for freedom once again.

In the song there is also a theme of things being better on the other side, if they get their freedom then the animals will live in perfect harmony for the rest of their life’s, which is not the case. The song represents a false assumption that life will be better if the animals are no longer ruled by humans.