The British constitution never looked less like itself than in the final months of the UK’s EU membership. In office was a government that constitutionally speaking should not have existed: one that could no longer command a majority in the House of Commons. Fearing parliament would legislate against a no-deal Brexit, the prime minister used the Crown’s prerogative powers to prorogue it for five weeks. This encouraged some opposition MPs to take to the Scottish Court of Session to petition for parliament’s right to reassemble. In September 2019, the Supreme Court ruled the prorogation unlawful, elevating the judiciary to a role it had not hitherto performed: as the ultimate guarantor of the British constitution’s conventions.
For those who saw the Court’s decision as correct and necessary, the bedrock principle at stake was parliamentary sovereignty. For many who had long wanted Britain to leave the EU, their inspiration was that very same principle. Each meant something different by it, but all deployed a caricature of the constitution—missing the awkward reality that the principle of parliamentary sovereignty co-exists in practice with an idea of popular sovereignty, centred on democratic consent to constitutional change. Paradoxically, it took the EU experience to teach British politicians about the British constitution.