'It is hard to predict how full Brexit would play out, because this scale of multiple simultaneous renegotiations of global trade agreements is unprecedented – and no country has ever left the EU. It certainly can’t be assumed that Britain is bound to get quick and good deals because it is a large economy.'

Deep into January 2019, as the Brexit clock counted down to mere days, an obscure and unnecessary World Trade Organization (WTO) rule was dragged into the heated public debate about Brexit...