Although initially a key driver behind the development of the EU’s Common Security and Defense Policy (CSDP) at Saint Malo in 1998, the UK has since come to be criticized for paralyzing this aspect of the European integration project. This paper draws upon a neoclassical realist analytical framework (Rose 1998; Taliaferro, Lobell & Ripsman 2009), which in contrast to Waltzian neo-realism allows room for domestic as well as international variables to be taken into account, to examine the UK approach towards the CSDP. This builds upon the work of Cladi and Locatelli (2012; 2013; 2015) in arguing that EU member states have been incentivised by the international system to engage with security and defence cooperation, but finds that the scope and nature of their engagement is impacted by unit-level intervening variables. This paper specifically identifies security culture (Kraus 1999) as one such important domestic factor through which UK involvement in the CSDP may be understood. It argues that this has helped shape the UK’s role with regards to the CSDP, where it has acted as both an important limiter and facilitator of its development since its inception. Furthermore, the paper finds that the core structural incentives for the UK to engage in European defence cooperation remain and therefore we may expect moves to solidify UK involvement in European defence post-Brexit, with the specific nature of this being conditioned by domestic pressures including security culture.