Arms' length bodies are often seen as a tool of technocratic governance, designed to insulate decision-making from the politicizing pressures of populist influence. This article examines a subset of arms' length bodies in the UK which challenge this convention: agencies which exist to 'champion' the voice of patients and the public in the four NHS systems (England, Northern Ireland, Scotland and Wales). We compare the functions of these agencies on paper and through qualitative interviews in each system which focused on public involvement in major service change (such as closing hospitals). We found that agencies in all four systems had struggled to demonstrate their legitimacy, squeezed between the demands of the elected Governments they answer to, the NHS organisations they are meant to support and challenge, and the publics whose voices they are meant to amplify. We argue that the evolving solutions found in each system demonstrate a foundational tension between locally-legitimate actors and nationally-capable political savvy.