How national parliaments adapt to the European Union is an important debate. However, scholars often overlook the regional aspect. This is particularly so for the UK where, despite devolution since 1999, scholarship remains largely devolution-blind. It is assumed that evaluating UK parliamentary adaptation only requires assessing the work of Westminster committees. This article takes a first step towards rectifying this oversight through reconceptualising UK–EU parliamentary engagement as multi-territory, not state-centric. This is demonstrated by comparing the social construction of practices in Scotland, Wales and at Westminster since 1999. Acknowledging devolution, however, does not just require comparing practices. Additionally, the paper asks how the ideas of devolution have been taken up by actors, potentially transforming the meaning of UK engagement for them. This necessitates new approaches drawn from interpretivist and constructivist institutionalist theories. Ultimately, therefore, the paper goes further than arguing for devolution-aware research to promoting change more generally in how parliamentary adaptation is theorised.