In April, Britain’s chattering classes became obsessed with the cost of the wallpaper in Prime Minister Boris Johnson’s Downing Street flat. New leaders are entitled to 30,000 pounds annually from taxpayers to decorate No. 11, where prime ministers since Tony Blair have lived because it’s larger than the flat above No. 10. Theresa May, Johnson’s predecessor, had given the space a modest makeover. Before her, David Cameron and his aristocratic wife, Samantha, had gone for a Scandi-style look. Boris and his fiancée, Carrie Symonds, opted for dark-green walls. The curtains were green, as were the window frames. Paintings replaced bookshelves. A…