March 27, 1625 — Charles the First becomes King of Britain

Charles the First, destined to end his rein several inches shorter than he began it, was a firm believer in his divinely ordained autocratic rights as King of England, Scotland and Ireland.  (He also claimed to be King of France, although even a claim of King of Calais would have been inaccurate, the English having lost their last French possessions in 1558.)

Charles would spend his entire reign battling his own Parliament, with an increasing lack of success, to maintain what he saw as the right and proper prerogatives of the King.  Reign and battle both would culminate in 1649, when a revolution led by Oliver Cromwell first deposed, then executed King Charles I.

King Charles I after original by van Dyck.jpg
By Follower of Anthony van Dyckshe-philosopher.com(original upload)
Sothebys 2012 (higher resolution upload), Public Domain, Link

As mentioned in:

Oliver Cromwell — Monty Python

January 6, 1649 – The Rump Parliament appoints a High Court to try the King

The Rump Parliament was what remained of the British Parliament after Colonel Pride had purged it a month earlier, leaving only those parliamentarians who supported the army.

On January 6, 1649, the Parliament appointed a total of 135 men to constitute a High Court for the trial of King Charles I for tyranny. A quorum was declated to be twenty of these appointees.

The trial of Charles I commenced shortly thereafter, and duly returned the guilty verdict it was intended to.

Colonel Thomas Pride refusing admission to the Presbyterian members of the Long Parliament.
By UnknownThe Tudors and Stuarts, by M.B. Synge, Public Domain, Link

As mentioned in:

Oliver Cromwell — Monty Python

January 30, 1649 — King Charles I of England is executed

It is the decisive exclamation mark that ends the English Civil War. Never before had an English monarch been deposed, tried and convicted of high treason, and then executed. (To date, no other English monarch has suffered the same fate, either.) The decapitation of Charles the First made plain to the people of England and the courts of Europe that the winds of change were blowing in England.

Charles’ son, Charles II, would eventually be restored to the throne that was his by right of primogeniture, and in the interregnum that followed, England would be variously led by Parliament, by Lord-Protector Oliver Cromwell, and briefly, by Lord-Protector Richard Cromwell (Oliver’s less talented and determined son). The restored king was a damned sight more careful of Parliament, and the gradual decline of the power of the monarchy would only continue from this time onwards.