The article analyses the renewed importance of bilateralism for the UK’s engagement with individual EU member states in relation to security and defence policy. By systematically scrutinising the bilateral agreements with 18 EU countries concluded between the EU membership referendum in 2016 and the end of Boris Johnson’s premiership in 2022, we argue that the United Kingdom currently finds itself in the process of transitioning from one policy regime (multilateralism) to another (bilateralism); we try to make sense of this strategy by looking at it through the lens of four key aspects stemming from regime theory, namely (1) triggering factors; (2) institutional design and adaptation; (3) path dependency; and (4) regime sustainability. The analysis shows how the sustainability of a purely bilateral regime, with its high degree of customisation and intrinsic reliance on specific reciprocity, is precarious, albeit while leaving open the possibility to incorporate a future multilateral component.