Making Differentiation Work: Criminal Justice Cooperation in the EU–UK Post-Brexit Framework

This article investigates how the European Union (EU) and United Kingdom (UK) have managed to maintain close criminal justice cooperation after Brexit, despite the UK's uneven integration with the EU, best described through the frame of differentiated integration (DI). While much attention is often given to the UK's various opt-outs in this field, the argument here is that the UK, prior to Brexit, shifted from a selective participant to becoming a more engaged partner that actively collaborated within EU judicial frameworks, recognizing the concrete benefits of joint legal and procedural work. The study shows that professionals on both sides not only advocated for retaining as many mechanisms as possible, but also diligently applied the agreed measures and addressed emerging challenges collaboratively. A conceptual discussion illustrates that in this domain, the UK represented a case of aligned DI, where constructivist influences and functional demands fostered deeper convergence with the EU, which persisted beyond its formal membership. The perseverance of judicial cooperation is thus not accidental but reflects a strategic recalibration by the UK, making EU disintegration difficult while fostering its external differentiated (re-)integration.