With Lizzie's record breaking moment yesterday I asked myself when the monarchy came in the position it is today, where the monarch or the royal family may not even influence politics or politicians.
I know, that British Kings and Queens were no absolute rulers and were dependent on the Houses of Parliament. But who was the last one introducing new taxes (or telling the Exchquer to do so), who was the last one spending public money for infrastructure projects (like rebuilding London for example).
Was there some acutal power left when Victoria ruled? George III? Queen Anne?
Legally, the British monarch has enormous constitutional power. To her is given the tasks of selecting the First Lord of the Treasury and his ministers, declaring war and making peace, giving Royal Assent to bills, dissolve Parliament, and a host of other power over the constitutional devices that British democracy requires to run. She is the third part of Parliament, along with the Commons and the Lords.
Now, the monarch does not exercise these powers at will, rather upon the advice of her ministers and MPs. The British constitution contains numerous conventions that 'govern' how the monarch uses his or her power - for example, it is a constitutional convention that the monarch will select as their Prime Minister the man or woman best able to command a majority in the Commons, which means, in essence, the leader of the victorious party in the most recent general election. However, these are not law. Were the monarch to ignore them, there would be no legal mechanism to restrain her power, no way to seek judicial review, and no way to legally remove her from power. There would of course be a constitutional crisis, but the Queen would not have acted unlawfully, merely unconstitutionally. These are not synonymous in UK law, as they are in, for instance, American.
So the premise that the monarch has lost their legal power is not necessarily sound; the British constitution still provides for immense legal power on the part of the Crown. Even were to accept that the monarch does not deploy her legal powers at will, it should be noted that the monarch does still receive the Prime Minister every Wednesday at 6:30pm, and so still exerts a considerable indirect influence on policymaking.
However, this is not r/askbritishconstitutionallawyers so to provide a historical perspective: the power of the monarchy was rarely settled in English and later British history. For instance, the Magna Carta, although very short lived, was one attempt to reign in royal power - though not, as American mythmakers suggested, to subject the king to the people. The principle that the monarch cannot govern without the consent of the rest of Parliament was established during by the Bill of Rights 1689, passed after the so-called Glorious Revolution.
Since that date, there was a general trend of granting more decision-making power to the common people, as opposed to the aristocracy and monarchy, and this process incorporated the Reform Acts, and ended in the granting of universal suffrage in the United Kingdom, which was achieved by the Representation of the People Act 1928.
However, during this period and since, the monarch has used their powers; when Asquith tried to pass the People's Budget of 1909/10, the Lords resisted. Asquith's plan was to advise the King to appoint more Liberal peers to break the Conservative majority in the Lords. The King, however, forced Asquith to fight another general election before he would do so, and that election cost the Liberals their majority in Parliament and forcing them into a minority government supported by Labour and irish nationalists. George V would select Stanley Baldwin over Lord Curzon based entirely on his own belief that the Prime Minister should be an MP and not a Lord.
Twice during Elizabeth's reign, she has effectively chosen the Prime Minister as the majority Conservative party has found itself unable to select a leader of its own; in 1963 the senior leadership of that party asked the Queen to appoint Sir Alec Douglas-Home as PM, almost nobody knew of this choice outside the Royal Household and the senior ranks of the Parliamentary Tory party. Before that, in 1953, the Queen appointed Harold MacMillan on the advice of WSC and the Marquess of Salisbury, who took an informal straw poll of Cabinet ministers.
The TL;DR of your question is that: the British Monarch retains enormous, though not absolute, power, which is exercised according to traditional, not legal constraints. The absolute power of the monarch was abolished in 1689, but the monarch has continued to wield some serious influence.
Main source is Turpin and Tomkins British Government and the Constitution.
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