Britain passed a major landmark on the road out of the European Union this week. But the terms of the latest agreement invite the same question that Theresa May was asked by a foreign journalist after her most recent speech on the Brexit negotiations – “Then why leave?” In almost every respect Britain is worsening its trading position, is giving up power and influence, and seeking a deal which would be as close as possible to staying part of the EU family while outside the single market and customs union. Why not just stay in?