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
The Executive which puts laws into effect and plans policy
The Judiciary, which decides on cases that, arise out of the 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 (theEuropean 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 MP’s in the Commons) yet he is also the leading member of the executive.
Also the Lord Chancellor is a member of the cabinet and therefore of the executive as well as being head of the judiciary.
The House of Lords also has a right to vote on bills so they are part of the legislative but the Lords also contains the Law Lords who are an important part of the judiciary.
As with the PM, the members of the Cabinet are also members of the legislative who have the right, as a Member of Parliament, to vote on issues.

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 (President’s office), the Legislative (Congress) and the Supreme Court (Judiciary) have very clear powers stated in the American Constitution that restricts each section’s powers and avoids crossover between the three sectors of politics.