None of the existing models for the future trade policy relationship between the UK and the EU come with a predetermined foreign and security policy relationship. This article assesses how the future EU-UK foreign and security policy relationship might be organised post-Brexit. It provides evaluation of the current EU-UK interrelationship in the fields of the EU’s Common Foreign and Security Policy (CFSP) and Common Security and Defence Policy (CSDP) and assesses the degree to which the UK is presently integrated into EU decision-making and implementation. It highlights that the UK needs to determine the degree to which it wants autonomy or even divergence from existing EU policies. The article concludes by rehearsing the costs and benefits of three possible future relationships between the UK and EU foreign, security and defence policy: integrated, associated or detached.