Amusingly, Britain chose both - losing the Empire and losing herself.
The process isn't over yet - you'll lose Ireland to the Irish, Scotland to the Scots as well.
The only sad thing about Britain's decline is that they chose it.
Britain did not
lose the Empire, it was unsustainable, and was more existent in theory than reality by the 1920's.
Ireland already is Irish. If you mean Ulster, then I very much doubt that anything other than the status-quo will happen there for the next three decades or so. Plus the Stormont Assembly still has a Unionist plurality.
The chances of Scottish independence are around 15%, if that. More likely is a 'devo-max' solution and a federalised Britain, though this won't occur under a Conservative administration.
So, how does Britain
choose decline? Seriously. I just don't follow.
The only sad thing about the decline of the United States is that they chose it.