Whilst the idea that 'Britain doesn't do revolutions' is somewhat untrue, there isn't a single revolutionary moment which can be pinned down as the end of monarchial power in Britain and beginning of parliamentary democracy free from royal interference. Instead it was a long process with a number of periods where monarchial power was significantly reduced and significantly increased.

The Peasant's Revolt in 1381 is an early example of a direct challenge to the monarchy's power, but of course it failed. There are many other notable and less notable rebellions against the monarchy as well. The Magna Carta signed in 1215 was, for many centuries, seen as the beginning of an erosion of the monarchy's power and held up as a great source of rights and liberties for the people of England, later Britain as a whole, against the absolute power of a monarch. Its role however was, and still is to a certain extent today, overstated. It guaranteed rights only to noblemen, aristocrats and barons rather than the people as a whole and was a peace treaty at heart rather than a document of political or civil rights. Despite this it is still an early document restricting the abilities of the monarchy.

The English Civil War, (I refer specifically to the English war here rather than the other related but connected British conflicts), saw turmoil for the monarchy. Royalist forces were eventually defeated by the parliamentarians and Charles I was executed, bringing about the first and so far only republican period in English and British history: the Commonwealth of England. Though the monarchy was gone Cromwell effectively ruled as Lord Protector with the old monarch's powers and there was not great change in the power structure of England as, say, the levellers had imagined and called for. This is contentious of course-Cromwell's Commonwealth was different from Charles' Kingdom and views of Cromwell vary a great deal amongst historians. Regardless however Cromwell's death and his son's poor leadership eventually meant the end of the republic and restoration with Charles II so the monarchy continues in a powerful role.

The Glorious Revolution of 1688 is seen by Whig historians of the nineteenth century, and their neo-Whiggish successors even to this day, as the defining transition to parliamentary democracy for Britain. Catholic James is overthrown by Protestant William, and Mary, on the basis of the new monarch signing the Bill of Rights in 1689. Whilst the monarchy was still powerful and important the Bill did restrict the powers of the monarchy and ensure the powers and rights of Parliament from interference from the monarchy.

The 1688 'bloodless' revolution, (there was a little bloodshed), is an extremely important date in the loss of monarchial power to parliamentary power-though not power for the vast, vast majority of people of course. Despite the Bill of Rights restricting their power the successors of William and Mary continued to influence political decisions for some time. Yet as monarchs came and went so too did their real hard power. Soft power and influence remained and the symbolism and figurehead role of the monarchy became increasingly important.

By the reign of George IV, both during his regency for his father and his actual reign, His Majesty's Govt. were the ones really running the show. During the Napoleonic Wars Prime Ministers began becoming the national leaders over the monarch-though of course George III's illness did not help. When George IV took over this trend was hardening. William IV got involved in the debate over parliamentary reform in the 1830s directly, whilst Victoria also caused a brief crisis with the political appointments of her ladies in waiting, better known as the 'bedchamber crisis' but the monarch's role was by now, largely symbolic. There have been crises and political problems involving the monarchy since then-see the Abdication Crisis in 1936 for example, but from the mid nineteenth century the monarchy's old powers were gone in practice, Prime Minister and Parliament exercising them 'on their behalf.'

Sources: The English and Their History-Robert Tombs

The English Revolution 1640-Christopher Hill

Revolution: The Great Crisis of the British Monarchy 1685-1720-Tim Harris