I don't think the Britons are really ashamed of their Empire. You see their left decry "American imperialism" but they speak of a different tones when it comes to Joseph Chamberlain. They hate what happens now because they aren't the boss like their grandfathers were.
The thing about Britain is that it could always change when change was necessary. It did not resist change and experience revolution as a consequence. When it came to tolerance, Britain was ahead of the rest of Europe. Hence in the late nineteenth century, European radicals such as Karl Marx, Kropotkin and other sought refuge in Britain. As for the slogan "American imperialism" - just a cliche from the left wherever you go.