After having consummated Brexit and defined a new British security and defense strategy, London and Brussels face the need to continue collaborating in a world characterized by a growing competition great power competition. Although the United Kingdom has chosen to prioritize its freedom of action by strengthening its special relationship with the United States, radically reforming its armed forces and drawing up a more dynamic foreign policy, the European Union is and will always be an inescapable step on its way, as demonstrated by forums such as E3 and EI2. Without a definitive framework that establishes the rules that will govern the relations between both actors in the field of defense, and taking into account the existing links in industrial matters and the perception of risks and threats, Brussels and London will have to establish bridges and find an accommodation for British participation in mechanisms such as PESCO. The article will focus on the definition of the post-Brexit British defence and security policies and the remaining open issues in this field with the European Union.