Princess Kate’s longtime assistant Natasha Archer left the royal household last year to start her own personal styling business, and she showed her support for one British brand during the Chelsea Flower Show this year. Archer was pictured in a group photo shared by St. Clair London on Instagram after the brand’s Chelsea pop-up, and a member of the Royal Family also made an appearance. Lady Amelia Windsor , whose grandfather is the Duke of Kent, wore one of St. Clair’s new season dresses in a photo included in the Instagram carousel. The model, who was once dubbed the most beautiful royal by Tatler , paired her sleeveless, square-necked blue floral dress with a pair of chunky black sneakers as she posed with St. Clair founder Blaire Donald. Lady Amelia Windsor (right) wears St. Clair's new pastel blue Lily dress. (Image credit: St. Claire London/Instagram) St.…