The separation of power is an integral part of American Politics but is less clear in British Politics primarily as one, the American model, is guaranteed in their Constitution while the British Constitution is uncodifed and therefore roles have merged between parts of government.
Government functions through three bodies:The Legislature which makes laws
In America all three branches are systematically split between the Executive (the president), the legislative (Congress) and the Judiciary (the Supreme Court). The president cannot serve in Congress when president and serving Congressmen cannot be a Supreme Court judge. In theory, no branch becomes more powerful than the other two so that a balance occurs. The American Constitution clearly states what the executive, the legislative and the judiciary can do.
In Britain this is not so clear. The legislative aspect is Parliament where laws are passed; the executive (which plans prospective laws and formulates policy) is the cabinet of the government and the judiciary is the Law Lords and the Judicial Committee of the Privy Council who have a final say on legal issues (the European Court excluded).
However, whereas the American model has separation as part of the American Constitution, this is less clear in Britain.The Prime Minister is an active member of the legislative (and can vote in Parliament, though a recent criticism of Tony Blair and Gordon Brown is that their voting record is one of the poorest of MPís in the Commons) yet he is also the leading member of the executive.
Therefore, there is a merging of roles in the British model. Some have argued that this is needed for flexibility in a modern society. Supporters of the American model claim that a written constitution gives a government the rights it has so that it cannot trespass onto power held by other parts of the political system or have its powers trespassed on by others. The Executive (Presidentís office), the Legislative (Congress) and the Supreme Court (Judiciary) have very clear powers stated in the American Constitution that restricts each sectionís powers and avoids crossover between the three sectors of politics.