The trilateral Comprehensive Security Integration and Prosperity Agreement (C-SIPA) signed between the United Kingdom, Bahrain, and the United States in June 2025 has granted the UK's military presence in the Persian Gulf an institutional and multilateral character. This article seeks to answer the following question: what mechanisms has this agreement created to enhance the UK's presence, and what are its implications for the balance of threat and Iran's defense strategy? The research hypothesis holds that the institutional linkage of the UK's naval base in Bahrain with the joint operational framework increases collective deterrence against Iran and limits Tehran's strategic freedom of action. The study employs a directed documentary content analysis method based on official documents and think-tank reports. The theoretical framework combines Walt's balance of threat theory with Buzan's securitization theory. The findings indicate that three factors have raised the cost of any confrontational action by Iran: the reduction of strategic ambiguity and the clarification of quasi-alliance commitments, the integration of surveillance and defense systems, and the discursive construction of Iran as the "threatening other." This research emphasizes the necessity of establishing a permanent national-level threat assessment working group and suggests that future studies examine the role of other European powers, particularly France, in the security architecture emerging from this agreement.