On his return, and upon the recommendation of Admiral Wilson, Dreyer was promoted commander and appointed Assistant to the Director of Naval Ordnance (DNO), John Jellicoe. At the end of 1907 he assisted in the trials of Arthur Hungerford Pollen's "Argo" rangefinder mounting and plotter on the cruiser HMS "Ariadne". He returned to the Admiralty, under the new DNO Captain Reginald Bacon and remained there until 1909, when he was appointed commander (executive officer) in the new dreadnought HMS "Vanguard", then completing in Barrow-in-Furness. In 1910 Dreyer was invited by Vice Admiral Jellicoe to be his flag commander, first in HMS "Prince of Wales" (flagship of the Atlantic Fleet) and then in HMS "Hercules" (flagship of the 2nd Division of the Home Fleet).