LONDON — On the eve of the tenth anniversary of the vote to leave the European Union, millions of British households are opening notices of a 13% energy price hike. The immediate cause is a geopolitical shock; Iran’s blockade of the Strait of Hormuz has choked off a fifth of the world’s oil and gas, but the deeper, more uncomfortable story is one the political class has spent a decade avoiding: Brexit itself has made Britain poorer, less secure, and structurally more exposed to exactly this kind of crisis.