This article examines the complex interactions between British national identity and the territorial identities of Northern Ireland and Scotland. We argue that the current literature on national identities in Britain misunderstands the nature of British identities in Northern Ireland and Scotland. Indeed, much of this literature wrongly defines Unionists in both of these areas. By examining the content of British national identity, a comparison of Scotland and Northern Ireland reveals that Unionism finds political significance through an ideological project committed to the Union. However, we also have to account for the differences in the Unionist ideology of Scotland and Northern Ireland. We argue that the institutional framework in which these identities and ideologies are exercised explains this variation. Overall, we argue that the debate on nationalism in the United Kingdom has not adequately shown how the integrative functions of British national identity can co-exist with the separatist nature of territorial national identity.