Earlier drones which were conversions from real aeroplanes, such as the Firefly, Meteor and Sea Vixen, had either 'U' (e.g. Firefly U.8) or 'D' (e.g. Sea Vixen D.3) designations. Whereas purpose built drones such as the Jindivik did not.
I don't see any need to give drones any specific designation - neither is there any precedent.