The Princess of Wales and Prince Williams officially ushered in the British summer social season on Friday. Gathering at Buckingham Palace over tea, finger sandwiches and sponge cake, Kate Middleton returned to a foolproof fashion formula: polka dots. The annual garden parties invite members of the public to recognise and reward their service, from charity volunteers and military personnel, to diplomats, academics and community champions. Prince William stepped in for King Charles, meeting and chatting to attendees in the palace gardens. For the occasion, Kate debuted a new custom-made Self Portrait dress, featuring a fitted white blazer and pleated polka dot skirt. The Princess has worn the British luxury label countless times over the years, while polka dot remains a staple of her summer wardrobe. At Wimbledon, she has presented the men’s finals trophy wearing an navy polka dot dress, and at a Kensington Jubilee street party, she plumped for a blue polka dot frock.…