In the WTS lore, the UK and the US make up the "Anglo-American World Power" represented by the 2 headed horned beast of Revelation and is considered the 7th World Power of Daniel. I find it helpful to try and see the WTS thinking which is not rooted in reality but their world view.
*** w12 6/15 pp. 16-17 Jehovah Reveals What “Must Shortly Take Place” ***
ANGLO-AMERICA AND THE TWO-HORNED WILD BEAST
12 Though the Anglo-American World Power is a mixture of iron and clay, the visions that Jesus gave to John show that this power would continue to play a key role during the last days. How so? John saw a vision of a two-horned wild beast that spoke like a dragon. What does this strange beast represent? It has two horns, so it is a dual power. John is again seeing the Anglo-American World Power but in a special role.—Read Revelation 13:11-15.
13 This wild beast promotes the making of an image of the wild beast. John wrote that the image of the beast would appear, disappear, and then rise again. That is exactly what happened to an organization promoted by Britain and the United States, one that was intended to unite and represent the world’s kingdoms. This organization appeared after World War I and was known as the League of Nations. It disappeared with the onset of World War II. During that war, God’s people declared that according to the prophecy in Revelation, the image of the wild beast would ascend again. And rise it did—as the United Nations.—Rev. 17:8.
14 John described the image of the beast as “an eighth king.” In what sense? It is not depicted as an eighth head on the original wild beast. It is only an image of that beast. Any power it has comes from its member nations, especially from its key backer, the Anglo-American World Power. (Rev. 17:10, 11) But it does receive authority to act as a king to carry out a specific task, one that triggers a chain of events that will change history.
*** w12 9/15 pp. 6-7 par. 13 How This World Will Come to an End ***
A prophecy in the book of Daniel enables us to establish where we are in the stream of time. Daniel describes an image in human form made up of different metals. (Dan. 2:28, 31-33) It represents successive world powers that have had a major influence on God’s people, past and present. They are Babylon, Medo-Persia, Greece, Rome—and the last, in our time, another world power. A study of Daniel’s prophecy shows that this final world power is depicted in the image as the feet and toes. During World War I, Britain and the United States forged a special partnership. Yes, the fifth part of Daniel’s image is the Anglo-American World Power. The feet form the last part of the image, indicating that no other human world power will appear on the scene. That the feet and toes are made up of iron and clay pictures the weakened state of the Anglo-American World Power.