Great Britain is only major country in the world which does not have a written constitution set out in a single document. The constitution in the UK is made up of Acts of Parliament common law and conventions which may be changed by general agreement.
The British Government sits in London. Great Britain is a limited or constitutional or parliamentary monarchy headed by Queen Elizabeth II now. The Queen acts only on the advice of her Ministers. She reigns but she doesn't rule. Parliament which consists of two houses: the House of Commons and the House of Lords both of which arose during the Middle Ages limit the power of the Queen. England was a first country to have a Parliament. The House of Lords is an out-of-date institution.
The House of Commons is elective and more powerful. The members of Parliament sit on both sides of the Speaker the government on his right the opposition on his left.
Most members of the government belong to the party which wins a majority of the sits in the House of Commons. The leader of the majority party becomes the Prime Minister. The Prime Minister chooses the Government. From the 60 ministers in the government Prime Minister chooses the smaller group of about 20 called the Cabinet. The Cabinet takes all major political decisions. At present there are four political parties in England the Conservative (or Tory) Party the Labour Party the Liberal Party the Social-Democratic Party.
Since 1867 British politics have been dominated by two major parties the Conservative Party and the Labour Party which both represent the ruling class.