When the U.S. Constitution was being drafted, its writers had the British Parliamentary system to draw on. The British system was the system they were used to and had learnt since childhood. However, because the monarchy was one of the chief things that the former colonists had rebelled against, any vestiges of monarchy and most forms of concentrated power were avoided.
Both the U.S. and British political systems have a head of state, a court system and an upper and lower house. Both have constitutions that lay out the rules for government and the rights of the people. Both systems are democratic in nature in that governments are put in place and removed from power by the will of the people and both have systems of checks and balances to limit the power of any one branch.
Head of State
In the U.S. political system, the president is the official head of state. The president is elected under the electoral college system. In the U.K., although the prime minister usually has the spotlight on political matters and is the official head of government, the queen or king is the official head of state. The queen officially signs off on acts of parliament and, just as the U.S. president delivers the State of the Union Address every year, the queen reads the "Speech from the Throne," which is written by the prime minister. In modern government, the monarch is more of a ceremonial figurehead and it is unusual for any member of the royal family to directly interfere with the political process.
The Upper House
The United States has a Senate as the upper house of the legislative branch and the U.K. has the House of Lords. Under the U.S. system, each state, regardless of size, has two senators. Originally, senators were appointed by the governor of the state they represented but they are now elected to serve six-year terms. The House of Lords is very different. Members of the House of Lords are not elected. The 792 members of the House of Lords are members by inheritance, appointment or their rank in the Church of England; they are not elected and cannot be removed by popular vote. Otherwise they serve the same purpose as the U.S. Senate. They discuss, debate and vote on legislation passed by the lower house of the legislative branch.
The Lower House
The U.S. House of Representatives and the British House of Commons have a great deal in common. Each house is made up of representatives elected by the people. In both systems control of the lower house goes to the party that has the most seats. Under the U.K. system, the leader of the party with the most seats becomes the Prime Minister and the official head of the government. Under the U.S. system this person would be the Speaker of the House. One other key difference is elections. Under the parliamentary system, the prime minister can go to the crown at any point and ask to dissolve Parliament. If this is done an election is called. An election can also be called if the prime minister loses "the confidence of the house." This means that the prime minister lost a vote in Parliament on a matter of confidence. Matters of confidence are usually, but not always, over budgetary matters. If the prime minister loses a vote of confidence, the end result is an election.