When Britain began imagining a post-imperial Commonwealth

At what point did the British Empire become untenable to maintain in its existing form?

British thinkers were already considering a Commonwealth in the late nineteenth century, well before the empire reached its maximum size.

Serious British recognition that the empire would change dates to roughly the 1870s and 1880s, when Commonwealth ideas began to appear. The empire reached its largest territorial extent after the First World War, but size did not mean policymakers saw centralized imperial rule as permanent. The issue was whether local government and a looser fraternity of nations offered a more workable arrangement. Many early proposals remained strongly London-centered and did not move at a pace colonized peoples would consider adequate.