Brexit has significantly impacted the UK’s environmental legislation, which was previously aligned with EU law. Despite promises of independent policy-making, UK environmental laws remain influenced by EU regulations. The EU-UK Trade and Cooperation Agreement (TCA) includes a “non-regression” clause to maintain environmental protection levels and re-balancing measures, promoting continued cooperation. This article examines the consequences of Brexit on UK environmental policy, focusing on the Retained EU Legislation (Revocation and Reform) Act and its sunset clause, which aims to adjust and retain EU laws. While the Environment Act 2021 (EA) establishes a new framework for UK environmental protection, there are risks of de-harmonisation. Challenges in implementing international environmental agreements, like the Aarhus Convention, necessitate new domestic compliance mechanisms. The UK government’s commitment to high environmental standards is evident in its proposal for an independent watchdog and firm stance in trade negotiations. Brexit’s impact includes recalibrating the UK’s relationship with international environmental organizations and reaffirming its commitment to global cooperation on environmental issues, particularly climate change. Overall, the analysis highlights the extensive effects of Brexit on UK environmental legislation in light of a thorough examination of the post-Brexit era.