HRH the Duke of Connaught was the order's Grand Master at the time, and after he had visited Jerusalem, Allenby was able to write to Wigram [Assistant Private Secretary and Equerry to the King
] on 5th May 1918,
"I had the unique honour of receiving, from the Grand Master of the Order of St. John of Jerusalem, the insignia of that Order in the Holy City itself; and His Royal Highness occupied, in the Kaiserin Augusta Victoria Hospice, on the Mount of Olives, the suite of rooms reserved for the Grand Master of the German Order of St. John of Jerusalem."
(from 'Allenby in Palestine
– the middle east correspondence of Field Marshal Viscount Allenby' selected & edited by Matthew Hughes, 2004)
The photograph below shows Allenby and others, just after they have received their awards at an investiture held by the Duke of Connaught in the Old City of Jerusalem. [Though as far as I can tell, the order being worn by Allenby in this particular photograph, is not that of St. John of Jerusalem]