Since the Brexit referendum, UK environmental NGOs have fought hard to shape the post-Brexit environmental law landscape. In this chapter, we argue that notwithstanding some clear weaknesses in the general place of legal expertise in UK environmental groups, the sector leant heavily on that expertise in advocacy around Brexit-environment, and with some success. Our research reveals three key weaknesses in legal expertise for advocacy in environmental NGOs: first, its scarcity; second, a patchy understanding of what it can do for a movement (beyond litigation); and finally, its lack of prominence in NGO leadership and strategic thinking. We identify two closely related features of the Brexit context that contribute to explaining both the insider, advocacy focus of environmental groups at this time, as well as the apparent paradox of a heavy use of legal expertise in that advocacy. First, political opportunities made demands on, and provided particular openings for, insider legally expert advocacy. This, we argue, created a specific set of what we call hybrid political-legal opportunities. Second, the post-referendum landscape created a space for intensive collaborative activity.