No documentary evidence can be found to substantiate the use of 'HM' in the title of the Office other than it being added to the name in the preface of the Almanac for 1907, which was produced in 1904 January. As Wilkins states , neither have we found any objection. My personal view is that with the growing international collaboration of Almanac Offices, started by Downing following the 1896 conference, (see also 1911) it was decided that this would be an obvious way to distinguish the British Office.
It must be noted here that the archives of HMNAO are incomplete; there have been several moves (12 to date). Wilkins notes that Eric Smith wrote: prior to 1926 little is known since in an unfortunate spring clean in March 1930 all past work and records were scrapped . Sadler also confirms  that the BAA required more storage space for its publications, and Comrie allowed them to use one of HMNAO's storage rooms. Sadler and Smith were given the job of moving the material, and Sadler notes that much was destroyed, including correspondence.