> The UK’s decline has taken centuries, though, and there are many events that one can point to. I think I agree with the grandparent’s comment about grandparents, about the arrogance of how things will continue to be a certain way.

I don't think that's accurate. I'd put the peak of the British Empire at just before WW1 and the obvious end of it somewhere between independence for India, Wind of Change, and Suez.

You could point to errors made before then, and IMO the Potato Famine was so egregious it was probably a contributing factor to the invention of literal Communism, but I'd say the actual collapse began with the Pyrrhic victory of WW1.