Most reasonable people accept that voting to leave the European Union has been a cost to the country so far, regardless of the long-term benefits that Brexiteers promise will emerge. Households are poorer since sterling’s collapse drove up inflation, growth is trailing other nations and the government has had to crank up borrowing. Less visible but equally damaging is the “opportunity cost” — the expensive trade-offs that have to be made as Brexit sucks all the energy from government.

Few like to think about the opportunity cost of their decisions. Tesco, for example, ploughed £1.2 billion into America before pulling out in 2013, shortly before it was engulfed in an accounting scandal. The supermarket group’s money would have been better spent on internal systems, it