In the UK, the lower house of parliament is also in charge of governmental bodies, as this web page shows.
Why is the UK Government/Administration and House all rolled into one entity? The head of the majority party in the house is the prime minister making him the head of government/administration. Why not have a separate administration/government and house such as happens in america?
The checks and balances of the UK parliamentary system are very different from the US, but they do exist. No answer here is doing to do better than give a summary.
The executive branch of the UK is essentially the Prime Minister and the Cabinet, together with some non-cabinet governmental posts. They are usually drawn from the majority party (or parties) in the House of Commons. However there is more separation than you would think in terms of powers. Executive functions continue even if parliament is suspended. There is a clear distinction between the powers given someone by virtue of their government office, and powers due to their election as an MP. And there are strict rules about separating government activities from parliamentary political activities.
For all the talk about "the most powerful man in the world", a Prime Minister of the UK arguably has more power within their country than the US President does in his. They can for the most part appoint their cabinet, declare war and run the country without any form of approval or confirmation from parliament. The powers of the cabinet are quite wide. Moreover they have a huge degree of control over the legislative agenda. The vast majority of bills in parliament are introduced by the government, and opposition typically spends its time opposing the agenda of the government rather than pursuing their own.
The main check on this is however an equally huge one. At any time parliament may introduce a 'vote of no confidence'. Such a motion is essentially a vote to fire the government. In US terms, this would be the equivalent of Congress being able to vote to oust the President whenever they wish. Clearly the Prime Minister can only govern with the support of the legislature, and it is this that prevents possible excesses by the government. The fact that the government is drawn from the legislature is unimportant - there are typically on a few tens of members of the government, and the legislature always outnumbers them.
I would imagine your question is akin to asking "Why is the US President separate from the House?" It is because our Constitution defines the executive role as being separate from the legislative.
The UK's common law simply doesn't make that distinction.
The office is not established by any constitution or law but exists only as per long-established convention, which stipulates that the monarch must appoint as prime minister the person most likely to command the confidence of the House of Commons; this individual is typically the leader of the political party or coalition of parties that holds the largest number of seats in that chamber. Source
Why is the UK Government/Administration and House all rolled into one entity?
The short answer is: because that's the way it's turned out. The UK's system of government has evolved over a long period of time, rather than being planned from the start, the way the US system was.
Power started with the monarch, was later shared with the Lords, and then also shared with the Commons. Over time, Parliament (the Lords and Commons) took more and more power from the monarch. In the last century, the Commons also took away some of the Lords' powers.
As for why members of the Government are drawn from the two houses of Parliament: the role of Prime Minister evolved because the monarch needed someone who could get things done (and in the case, of the first person to be referred to as PM, who could also speak English), and this meant being able to have sufficient support in Parliament to pass laws.
The head of the majority party in the house is the prime minister making him the head of government/administration. Why not have a separate administration/government and house such as happens in America?
The British system came first, and has proven to be effective in a number of countries, so why change?
If you're asking what checks and balances there are in this system, then it's true that a government with a large majority in the Commons could, in theory, do what it wanted, including making drastic changes to the constitution.
In addition the comments by @Brendan about the selection process and legal distinction between the United States Constitution and the British parliamentary structure, it seems that this question attempts to get at the reasons for, and potential consequences of, the following question:
First, the reason for this construction is fairly easy to uncover given the evolution of government in the United Kingdom throughout history. Unlike the United States which had a hard and fast split from the United Kingdom and a deliberate period of establishing an entirely new government, the government of the United Kingdom has grown and evolved for centuries into what it is today. This gradual evolution from a tribal society, to a pure monarchy to the democracy we see today is the main reason the United Kingdom does not have a distinctly independent executive. Ever since the signing of the Magna Carta in 1215 the people of the United Kingdom have been trying to wrestle more power away from the monarchy and into the hands of the people. This led to the creation of Parliament with the power to pass some laws. Eventually, that power extended to all laws, including constraints on the monarchy. However, as the monarchy existed throughout this period, and continues today, even as the executive power transitioned away from the monarchy, the task of selecting a new executive came from the one established power source that existed, Parliament. This is the same reason that unlike many modern democracies, the United Kingdom does not have one constitutional document, but rather a collection of laws that together make up the basis for their government. The United States on the other hand had the opportunity to start with a clean slate, with the opportunity to lay out all of their institutions in the best way the Founding Fathers could think of, and they wanted an independent executive in order to help differentiate themselves from the system they had just rebelled against.
Determining the potential consequences of the executive being so intertwined with the legislative branch is a bit more nuanced. Both nations (the United States and United Kingdom) believe that the power of the executive should be held in check by a strong legislative branch and allow the legislature to pass laws limiting the power of the executive and even remove them from office (impeachment proceedings in the United States and a vote of no confidence in the United Kingdom). The real difference between the two systems is that the United States has a two party system and the United Kingdom a multiparty system. In the United States, where there are only two dominate parties and the majority has enough votes to pass legislation on their own the power of a legislative executive could be almost unlimited. However, in the United Kingdom where the majority party may not have an outright majority and coalitions between multiple parties both in opposition or in forming a government are often required, the power of the majority party is much less. This makes the power of the opposition to force a vote of no confidence a much more plausible event than impeachment proceedings in the United States and leads to a more practical check on the executive's power. For example, in the United Kingdom today the government is formed by a coalition of Conservatives and Liberal Democrats with a Prime Minister and Deputy Prime Minister from different parties.
In addition to the head of the government, the real work of the government in both countries is handled by cabinet secretaries and ministers and both countries have restrictions on these offices. In the United States, a person selected to serve in these offices must immediately resign any other elected office they hold. Similar rules exist in the United Kingdom as well as limits on how many can be selected from the House of Commons, House of Lords, etc. This ensures that the work of both branches of government can continue effectively and allows for a real distinction between the branches of government, despite the overlap at the Prime Minister level.
