Question: does this have any historical precedent or parallel? Has any other country, peacefully and uncoerced, divested itself of so much territory and income?
Let's talk about this premise that Britain divested itself of its empire "peacefully and uncoerced" because its begging the question.
The final dissolution of much of the non-white British Empire was only "peaceful" in the sense that final independence was won with the stroke of a pen, not the barrel of a gun, but that stroke of a pen was won with decades of revolts, thus the "coercion". And the transition to self-rule, particularly in the case of the partition of India and the Middle East, was quite sloppy and violent.
In contrast, predominantly white countries like Canada, Australia, and New Zealand, had peaceful transitions. They weren't treated as natives to be exploited because most of the natives were dead, but as British living in a far away land.
The major point of having an empire is to exploit it to benefit the home country. Trade, strategic raw materials (oil, metals, jewels, rubber), strategic positioning, and international prestige to name a few. Exploiting imperial possessions is great for the home country, not so great for the people living in the possessions, native or not. This basic economic tension belies the "peaceful" nature of Imperialism.
Over time, the possessions will start to develop, especially the largest and most prosperous. This is in the best interests of the locals, but isn't necessarily in the best interests of the empire. Therein lies the tension. A prosperous and sophisticated people will start to question why they're paying tax to some island they've never been to. And, with a very, very broad brush, we have the American Revolution which was not very peaceful and quite coerced Britain.
But that's ancient history, right? Britain peacefully divested itself of its major territories in the 20th century, right? Well, no. Instead, it was smart enough to let them go after enough wars had been fought, and it was clear Imperialism was done, and this was no longer a profitable venture.
1905 saw the end of the Russo-Japanese War. The tiny "backwards" nation of Japan soundly defeated Russia, a major European power, in a conventional land and naval war. People took notice, but more about the climatic naval Battle of Tsushima than the Imperial defeat. And anyway, they told themselves, Russia has always been a bit backwards herself. But the crack in the armor was there: the empires could be defeated.
Then WWI happened, the first "world war", because it didn't just happen in Europe but also at their imperial possessions. It saw millions of colonial troops, some native, some colonists, fighting all around the world for their empire, gaining prestige, and gaining their own sense of nationalism. It saw FOUR empires toppled: Russian (defeated in war and taken over by communist revolution), Austro-Hungarian (disintegrated into nations), German (stripped of her colonial possessions and some home land), and Ottoman (carved up into imperial possessions).
The resulting chaos would take a whole book to explain. The Empires could be defeated, and the possessions had a new sense of pride and nationalism. In some cases they were promised self-determination... and then it was snatched away. In particular, the Arabs were enticed to revolt against the Ottomans with the promise of freedom... which was taken away at the end of the war, their territories carved up into the arbitrary kingdoms we still have today by the French and British.
The following 20 years after the war would see more fighting as the Allies tried to keep the lid on the Imperial system. They'd fight the Turks in Anatolia (unsuccessfully), the Communists in Russia (unsuccessfully), the US had its Banana Wars, and Britain was suppressing revolts, mutinies, and protests in India to name a few. The locals were becoming better armed and organized, and the empires were becoming costly to maintain.
Then WWII happens. The war in Europe weakens the colonial powers and the Japanese take advantage whipping FOUR empires: Britain, France, US and Dutch Pacific possessions all fall. Britain relies heavily on its Indian troops further spurring nationalism. Regardless of the ultimate outcome the message is clear: the empires may not always defend us, they can be defeated.
The end of WWII saw everyone but the US exhausted. The Italians were defeated, and with them went control of their empire. France had to rebuild, yet still had dreams of empire and would fight to try and keep it. Britain had most of its empire intact, but it wasn't the same, particularly in India where all through the war they had to keep the lid on growing unrest over self-rule.
More mutinies and revolts in India followed WWII and eventually independence was won, but the partitioning and mass migration of peoples due to the freshly drawn borders would lead hundreds of thousands dying, followed by many more dying in decades of war between the new countries.
Britain divested its territory "peacefully and uncoerced"... if we only look at the white-dominated territories, or define "peaceful" to mean "Britain didn't lose an overt war". If Britain did so, then so did France, and the US.
I don't mean to bag on the British Empire, but on the idea that it was somehow different from the rest. I also don't mean to bag on the questioner, this is a really pervasive myth. While Britain did manage its dissolution better than most, empires are messy and exploitative things. The more you look into them, the more of the mess and exploitation you see.