In the United Kingdom there is conventionally no such thing as British Nationalism. The British recognise the nationalisms of others but not their own. First there were the anti-imperial nationalists, Irish, Indian, African but there were also potentially dangerous continental European nationalisms, above all the German, and also Latin American ones. More recently nationalism in the British context has meant Scottish or Welsh nationalism, and the spectre of English nationalism. In this lecture I want to suggest that the United Kingdom, just like Ireland and India, had its own post-imperial, indeed anti imperial nationalism, though one which could not speak its name. This nationalism, which flourished from 1945 to the 1970s, was economically as well as ideologically nationalist, and manifested itself in the internal rebuilding of the nation, and a national and anti-imperial historiography with 'decline' at its centre, concerned to make a nationalist critique of British capitalism. This argument is at the centre of my new history of the United Kingdom in the twentieth century, which thus challenges the prevailing assumption that the fundamental story is one of liberal imperialism turning into a weak social democracy transforming into neo-liberalism. We need, I suggest, to apply to the British case what is obvious in the history and historiography of the rest of the British Empire.