Andrew Walker: A UK-EU trade deal is another WTO issue

So the UK, it seems, is headed out of the European Union's single market, perhaps also out of the customs union. Prime Minister Theresa May has said she wants to preserve barrier-free trade between the UK and the EU as far as possible. One option that has been floated, if the two sides can't agree a comprehensive free trade agreement, is sectoral deals. They might cover cars, for example, or perhaps financial services. But there is a problem with this approach: World Trade Organization rules.