Well, just like the title says really. William Canendish was the husband of Kathleen Kennedy (JFK's little sister) and ran in a by-election in 1944 for Parliament. Let's say he wins that, but what course would be need to chart through British politics to become Prime Minister? Bonus points if he's PM while JFK is President (no Kennedy assassination is fine to meet this requirement).
Also, any idea's what his government would look like or what would be done?