By Isobel Pankhurst & Joseph Wilkes Prince William’s confession about a significant secret he kept from Kate Middleton continues to intrigue us, as do all matters related to Royal marriages and engagements. The Duke and Duchess of Cambridge, who will be celebrating their 13th wedding anniversary on April 29, are no exception. Royal expert Robert Hardman describes the announcement of a Royal engagement as the “starting gun” for months of anticipation, which was certainly the case for the Waleses. “When a royal couple tell us they’re engaged, we’re fascinated by everything from the dress design …