The market recently was packed with tourists taking advantage of the British pound’s slide against other currencies in the wake of the country’s vote to leave the European Union. But the vendors were not celebrating. “Brexit” has sown deep uncertainty in Britain’s food system, which for the last 43 years has been entwined with the rest of Europe’s, relying heavily on the EU for everything from pork to peaches to farm subsidies to the labor that picks its tomatoes. Now, the country is going to have to rethink how it feeds itself, from farm to fork. But what Britain can’t do is feed itself.