The political system of the United States is basically imitated by the British system. Since the United States did not have a king before, it did not imitate the British constitutional monarchy.
The United States is a capitalist country. Its highest authority is the president, but the president is not completely in charge. The U.S. Congress decides government affairs. In other words, what the president decides to do, Congress decides whether to do it or not.
The United Kingdom experienced the Industrial Revolution in the 1860s. The power of the king has been completely confiscated, and its highest power is the prime minister. In addition, the United Kingdom also has a Congress, whose power pattern is similar to that of the United States.