Imperialism and multiculturalism

I posted in March about a history of multiculturalism by Alain Finkielkraut which goes back to the anti-Enlightenment reaction of Johann Gottfried Herder and his rejection of absolute principles in favour of regional ones. Well, I was looking again at Boris Johnson's The Dream of Rome and was struck by a contrast that he makes between the Roman and British Empires.

... the Romans assimilated successfully and created a universal sense of Romanness; and for reasons of racism, religion and cultural prejudice on both sides, the British have failed to create anything like a comparable sense of Britishness either abroad or, indeed, at home. Now we are dealing with the consequences, in Britain, of adopting that communalistic approach, as the children of our imperialistic possessions grow up, in our own cities, in a way that is often balkanised and alienated. (p51-2)

Do you think the Guardianista supporters of multiculuralism acknowledge their imperialist inheritance?