Queen Elizabeth II appoints the United Kingdom's next prime minister while in Scotland, it will mark the first time in her 70-year reign that the appointment will happen outside of Buckingham Palace.A Palace source tells ET that Her Majesty will meet with outgoing Prime Minister Boris Johnson next Tuesday at Balmoral, her castle in Scotland. She will then meet with the new prime minister shortly thereafter, also at Balmoral. The source added that the queen's mobility issues are keeping her in Balmoral for now.