The State Opening of Parliament doesn’t usually offer much in the way of cutting-edge style, unless page-boy knickerbockers and buckled shoes are suddenly back in vogue. But on Monday, the Prime Minister’s girlfriend, proudly watching her beloved from the House of Lords gallery, put paid to that. She may have been dressed demurely in navy and tartan, but the oversized navy hair band framing her sweet, round face screamed high fashion. No society do of late has been complete without one. Whether it’s aristocratic actress Cressida Bonas in a bright red headpiece at Ellie Goulding’s Castle Howard wedding, or the Duchess of Cambridge at the St James’s Palace christening of her youngest child, Prince Louis, no posh girl worth her pink Himalyan salt has stepped out without a £300 piece of croissant-shaped fabric (prices can rise to an eye-watering £900) atop their shining heads. The Duchess of Cambridge wore a powerband at the St James’s Palace christening of her youngest child, Prince Louis, in July Aristocratic actress Cressida Bonas in a bright red headpiece at Ellie Goulding’s Castle Howard wedding Carrie Symonds, the partner of Prime Minister Boris Johnson, attends the State Opening of Parliament in the Houses of Parliament…