Well... 400 years back is 1614, or before the "Wars of the Three Kingdoms" (including the English Civil War), which was enormously destructive, bloody, disruptive and revolutionary.

Out of the aftermath of that, you've the Glorious Revolution of 1688, where a foreign army invaded and seized power, all be it with relatively little bloodshed and with local support. The aftermath of the overthrow of the Stuart dynasty leads to the Jacobite rebellions, including a substantial invasion of Northern England in 1745/46.

However since 1746 government of the UK England [1] has been relatively stable. One point of view is that England got its transition to a (partially) democratic constitutional monarchy out of the way some time ahead of the rest of Europe.

[1] (Edit: apologies, saying "the UK" would, over much of the time period in question, include Ireland, which is a whole other story)

That's not to say there hasn't been revolutionary threats; there was heavy suppression of radicals in 1794, the protests including the "Peterloo massacre" after the Napoleonic wars, the reform crisis of the early 1830s, the Chartist movement, anti-Corn-Law League and the cross-fertilization of the European revolutions of 1848.

There was however substantial, possibly even "Revolutionary" reform in this period; the Act of Union with Ireland in 1801, the Catholic Relief Act of 1829, the Great Reform act in 1832. All of these undoubtedly acted as a pressure valve for reform, particularly splitting the interests of the growing middle classes from those of the working classes.

Some (e.g. Ian Christie) argue that the social cohesion formed from the system of poor relief in the UK was an influential factor. Royle argues that this was part of an inherent social conservatism, which in combination with deft management from the political and social elites (including the Whig reform acts) and effective suppression of sedition meant that although Britain approached revolution it never went over the edge. It does however seem to have been a rather close-run thing, especially around 1832. It doesn't seem at all an implausible counter-factual that had the Whigs failed to pass the Reform Act then revolution would have resulted.

Since 1850 there seems to have been much less risk of a violent revolution than the period before that, but possibly somebody else could write about that.

References: Christie - Stress and Stability in Late Eighteenth Century Britain

Royle - Revolutionary Britannia?

I love this question. Some of the reasons, but by no means all of them I will give here.

One reason was that the UK got rid of absolute monarchy relatively quickly. We should note that long before the period that we are discussing here the power of the king was much more circumscribed than other European Monarchs. Quite often the king was forced to go cap in hand to parliament to get money to fight wars and it was the money raising efforts of Charles I outside the remit of parliament that led to the English Civil War. After the glorious revolution of 1688 the UK got a pseudo-democratic system of government where power was shared between the King, the House of Lords (HoL) and the House of Commons (HoC). Eventually the power of the king was diluted by both custom and legislation and was eventually vested in parliament as a whole. So despotism was no longer an option and there was thus no prospect of a coup by nobles because the king gave up his power voluntarily. Then, in the 19th century more and more power was taken away from the House of Lords, for example the Parliament acts of 1911 and 49 which took away the power of the Lords to veto "money bills". Thus power was taken from the nobles and was everything was devolved into a democratic process.

It is my belief that in many ways that social system in the UK was more flexible than for example France. The nobility was not completely cut off from the rest of the population. First off the Nobility was always taxed just like everyone else. So the tax burden was more evenly distributed across the population unlike France where the third estate alone bore the brunt of financing wars and suchlike. Intermarriage between nobles and commoners was relatively common, most often because the nobles were strapped for cash an would thus marry the daughter of a wealthy industrialist, conferring money on the nobles and social cache on the industrialist. One late example is Winston Churchill whose mother was the daughter of an American financier, her marriage restored the books of the Dukes of Marlborough. So the aristocracy was thus able to incorporate into itself the brightest and the best of the new bourgeoisie that might have otherwise become revolutionary.

Edit -- Spelling