Boom! Didn't think I'd manage this one. Started as the Duke of Normandy to form Britannia by Oct.17 1075, but I had to resort to some gamey tricks to get there. As the range of Holy Wars has been much reduced since 1.092, getting 200+400 piety to form your second kingdom and the empire itself is a chore. I circumvented this by gifting land to my clergy and when I ran out (of land) I banished Harold II and other prisoners from an earlier faction rebellion.
Not pretty, but effective!:
Not pretty, but effective!: