It’s the company’s Papilio style.
About the coat:
The Beulah London Papilio coat has an a-line shape, covered buttons and silk satin detailing. It’s made from a wool crepe fabric.
Kate and Beulah London:
Kate’s worn Beulah London for many years. She owns several coats, dresses and scarves from the British fashion brand. The company’s owned and operated by one of Kate’s friends, Lady Natasha Rufus Isaacs, who has known Prince William since they were infants.
Pictured below, Kate in Beulah London: