For me what made the country 'great' goes way back further than imperialism. As far as I'm concerned Britain is a mongrel nation at best, made up of bits of everywhere else. Sadly though ye olde knuckle draggers only want to see the other parts when the world map was largely pink!
I don't blame immigrants for anything, why not come here if there's a chance of making a better life? I do blame the racists etc for stamping their foot about immigrants coming here when all our ancestors were immigrants in the first place.