While there has been much talk of the role of parliaments and courts in the Brexit process, far less—indeed very little—has been said about the challenges facing the largest part of the UK government: the administrative branch. Whatever results from the UK’s negotiations with the EU, Brexit will likely necessitate wide-ranging and fast-paced administrative reform in the UK. In this article, we use a detailed case study of a particular part of administration—the Competition and Markets Authority (CMA)—to highlight the nature and extent of the challenges facing administrative agencies. This case study is demonstrative as, while there is an extant UK competition administration structure, competition law and enforcement is highly Europeanised. We propose that the challenge facing administrative bodies in the UK—including the CMA—can be understood as possessing three key dimensions: internal organisation issues; external coordination issues; and substantive legal issues. We argue that, in many instances, these three dimensions will be in tension which each other. That is to say, the reality of reforming administration post-Brexit will involve trade-offs between questions of internal organisation, external coordination, and substantive law.