Addressing the case of the UK's membership of the European Women's Lobby (EWL), this article explores the significance of European Union (EU) civil society organization (CSO) membership for domestic CSOs and, in turn, the impact of Brexit on (the nature of) that membership. Analysis adopts a territorial perspective to reflect the UK's 'four nations' approach to representation, which affords the constituent sub-state CSOs notable access to this European network. Analysis of interview and text-based data identified shared core beliefs and resource dependencies as binding the UK's sub-state CSOs within the EWL, resulting in capacity building at the sub-state level. These shared core beliefs can explain the EWL's decision to formally accommodate the UK coordination post-Brexit, with resource dependencies acting as a further incentive for sub-state actors. However, beyond the policy subsystem , there are practical constraints placed on the UK's ability to collaborate and a diminished value of some resource.