The British prime minister has so much power, why not overthrow the queen and become king himself?

it's easy. Royal power is not small.

First of all, British society is hierarchical, and there are differences between master and servant. Britain, Australia, New Zealand, Canada, Falklands and other British territories, 6% of the land belongs to the aristocratic class, overseas are nominal territories, but 7% of the land in the British Isles can really belong to the nobility. Hundreds of thousands of aristocratic classes control millions of square kilometers of land in the Commonwealth; The royal family has the ownership of many overseas territories, and the tax revenue is its own. Where there is money, there is capital. As the head of the aristocratic class, the queen has a lot of natural rights.

Secondly, the parliament is far apart. In the final analysis, it is optimates who controls the upper house. For example, generals, warlords, ministers of finance and admirals of the navy all have to be of royal or aristocratic descent to have capital, and these are all positions of real power. The House of Commons is the mirror of all kinds of folk politicians. You can bounce whatever you want. Without a title, you are nothing. This is painting cakes to satisfy your hunger. I don't see Beckham desperate to get a title.

furthermore, factional disputes need someone to make a final decision. Congress is divided into three parties. The Labour Party is a local warlord or entrepreneur with American background. The Conservative Party knows by its name that it is the mouthpiece of the royal family and its leaders are all nobles. The Liberal Democratic Party is the absolute stage for the activities of various politicians and speculators. Naturally, the main wrestling of the parliament is the confrontation between the Conservative Party and the Labour Party, and the parliament is the battlefield for the traditional aristocratic class to confront foreign capital forces. Most of the Labour Party has a hard time with the royal family and tends to join the European Union, while the Conservative Party insists on maintaining the British tradition and being a European troublemaker. The Queen tends to maintain the status of the Conservative Party, such as supporting Cameron, so that Britain can keep the pound and not trade directly with the EU, so as to maintain the power of the royal family and the Commonwealth.

finally, it is politics. The head of the Commonwealth is the queen, not the prime minister. The queen can dissolve the Australian government in minutes, but the prime minister can't. He doesn't have the power. Maintaining the status of the Commonwealth and economic circulation, maintaining the status of the pound, and maintaining the United Kingdom (Scottish independence and other issues) all need the Queen to maintain.