HINT: They had a "Special Relationship."
The answer is after the JUMP.
The men had a personal connection via marriage. Macmillan was the uncle-in-law of JFK's deceased sister Kick. Kick was the war widow of William "Billy" Cavendish, the Marquess of Hartington. He was the first born son of Edward Cavendish, the 10th Duke of Devonshire, whose sister was married to Harold Macmillan.
The familial connection between Kennedy and Macmillan seems weak: too attenuated to matter. Yet, it did, in part due to the influence of one of the 20th Century's most remarkable women. In the decades following Kick's death in a 1948 plane crash, JFK became close with Kick's sister-in-law, Deborah, the Duchess of Devonshire (née Mitford). The Devonshires were surprised and honored to be seated in the family section at Kennedy's inauguration, and Deborah was guest at the White House during the Cuban Missile Crisis. Kennedy even referred to the British leader as "Uncle Harold," in reference to Deborah's name for him, her uncle by marriage.