The UK offers a distinctive case study of the role of courts and Parliament 1 in a modern parliamentary democracy during a period of rapid change and uncertainty. The pivotal constitutional doctrine is parliamentary sovereignty that defines the basis of the UK’s constitutional arrangements. Sovereignty attributes to the UK Parliament unlimited authority in domestic matters by permitting Parliament virtually unfettered power. It also defines the relationship between Parliament and public institutions, prioritises political power 2 and sets the constitutional agenda. Most controversial is the UK’s continued membership of the European Union as well as the jurisprudence of the European Convention on Human Rights following the EU referendum decision to leave in June 2016. In general the courts operate within the limits of not being able to hold an Act of Parliament unconstitutional or illegal 3 because of obedience to parliamentary sovereignty. EU law is one of the few exceptions to the doctrine of sovereignty, conceding EU law has priority over UK law. In the case of Brexit and leaving the EU, the most significant constitutional case heard by the Supreme Court 4 relates to the role of Parliament in the decision to trigger Article 50 of the Treaty of European Union to withdraw from the European Union after the June 2016 referendum decision to leave.