For the last 150years there were only 2 parties: the Conservative Party and the Labour Party. A new party – the Liberal Democrats – was formed in 1988. Social Democratic Party is also the new one founded in 1981. Other parties include two nationalist parties, Plaid Cymru (founded in Wales in 1925) and the Scottish National Party (founded in 1934).

The effectiveness of the party system in Parliament rests largely on the relationship between the Government and the Opposition parties. Depending on the relative strengths of the parties in the House of Commons, the Opposition may seek to overthrow the Government by defeating it in a vote on a "matter of confidence". In general, however, its aims are to contribute to the formation of policy and legislation by constructive criticism; to oppose government proposal - it considers objectionable; to seek amendments to government bills; and to put forward its own policies in order to improve its chances of winning the next general election.

Because of the electoral method in use, only two major parties obtain seats in the House of Commons. People belonging to smaller political parties join one of the larger parties and work from within to make their influence felt. The exception to this are members of the Scottish National and Welsh Nationalist Parties, who, because their influence votes are concentrated in specific geographical areas, can manage to win seats although their total support is relatively small.

Her Majesty's Government: Prime Minister, the Cabinet. Her Majesty's Government is the body of ministers responsible for the administration of national affairs. The Prime Minister is appointed by the Queen, and all other ministers are appointed by the Queen on the recommendation of the Prime Minister. Most ministers are members of the Commons, although the Government is also fully represented by ministers in the Lords. The composition of governments can vary both in the number of ministers and in the titles of some offices. New ministerial offices may be created, others may be abolished and functions may be transferred from one minister to another.

The Prime Minister is also, by tradition, First Lord of the Treasury and Minister for the Civil Service. The Prime Minister’s unique position of authority derives from majority support in the House of Commons and from the power to appoint and dismiss ministers. By modern convention, the Prime Minister always sits in the House of Commons. The Prime Minister presides over the Cabinet, is responsible for the allocation of functions among ministers and informs the Queen at regular meetings of the general business of the Government. The Prime Minister's Office is situated at 11 Downing Street.

The Cabinet is composed of about 20 ministers chosen by the Prime Minister. The functions of the Cabinet are initiating and deciding on policy, the supreme control of government and the co-ordination of government departments. The exercise of these functions is vitally affected by the fact that the Cabinet is a group of party representatives, depending upon majority support in the House of Commons. The Cabinet meets in private and its proceedings are confidential. Its members are bound by their oath as Privy Counsellors not to disclose information about its proceedings, although after 30 years Cabinet papers may be made available for inspection.

So Great Britain is the constitutional monarchy. Monarch is the Head of the State. But Queen or King rules with the support of the parliament. And practically monarch have no real political power. The main political decisions are made by the Parliament and Cabinet. And the House of Commons are more powerful.