As a Soutie, I spend time in (and call home) both South Africa and England. The debate around decolonisation is viscerally real for me when I step off the plane on either end of the pond.
The discussion around Britain's colonial past is noticeably absent in British politics. Currently, there is far more National than Imperial discussion. The narrative of Brexit is 'taking back control'. Unless you are Scottish, then the talk is against 'divisive Nationalists'.
The driving powers behind the European Project were the former European Colonial powers. Since the second world war (which was largely a European led civil war that spread), with less global clout, focus shifted away from maintaining global hegemony to working together to avoid a repeat of the industrial scale human butchery. In 1917, the House of Windsor changed their name from Saxe-Coburg and Gotha because of anti-German sentiment. The Emperor of Russia and Germany were first cousins of George. Emperor Nicholas was forced to abdicate in 1917. Emperor Wilhelm abdicated in 1918. The post World War II story had then shifted from the evils of Imperialism to the evils of Fascism, and the new cold war.
It is worth noting that England itself has a population of around 55 million people (would make it the 9th biggest province in China). That is roughly the same as (the official) population of South Africa. So a true blue believer in one-person, one-vote, democracy should believe that the two countries should have an equal voice on the global stage. You can't push against elites in favour of 'the people' if you want 'your people' to be elites.
The first European global colonial power was Spain. Spain currently has a population of 46 million, and since the 1898 Spanish-American war has had to radically alter its view of its place in the world. This is hard. The former world power China, refers to the period between 1839 and 1949 as the 'Century of Humiliation'. If part of your identity is being a pretty big deal, it is hard when other people start doing well. As America became the world's dominant power, it too developed 'American Exceptionalism'.
Unlike wealth, status is zero-sum. A big part of Britain's narrative, and identity, is that of being leaders. It is a very uncomfortable conversation to start unpicking the less attractive parts of the past. It is far better for self-image to celebrate the good bits. The British aren't the first to have to re-establish a place in the world after being on top. They are some way down the road, and the older generation still have lingering memories of grandeur.
Perhaps we will one day be able to define ourselves in a way that doesn't require relative comparison.