Social integration is one of the most urgent issues in Western countries, where cultural diversity has been recognized to make social unity danger. In the past, social integration used to be tackled by multiculturalism, which was an effort to recognize cultural diversity as a positive aspect of society. However, multiculturalism is losing the moral support of society because multiculturalism itself could not be a social glue among different people; rather it is thought to make society unstable. Thus, a new philosophy and policy is required to manage the issues of social integration in a globalized social environment. In order to create social integration, on one hand, people with different backgrounds need to share the same concept of society. On the other hand, people have to become accustomed to cultural diversity. However, this is not easily accomplished because both measures for social integration seem to be politically contradictory and incompatible. How, and by what reasons, could these two conditions of social integration be satisfied? To answer this question, this paper scrutinizes the changes in social integration policy in post-war Britain. I divide social integration policy in Britain into three stages focusing on the relationship between social unity and cultural diversity: from after World War II to 1979; Thatcher's and Major's Conservative Governments; and Blair's new Labour Government. The social integration policy and philosophy of the new Labour Government in particular is important because it represents post-multiculturalism discourse for social integration. The Labour Government tried to establish social integration by introducing an abstract common identity, which both the majority and minority groups could accept and which is compatible with various cultural or religious conventions and teaching, as it were, citizenship and Britishness, as a set of liberal values. Although the Labour Government's policy itself was controversial, it is giving us a reference point for the debate on social integration in a post-multicultural era.