There is a report that the Grenediers want Kate to take over for Andy.Kind of weird since she has never served.
Colonel of the Grenadiers is a patronage, and not a real military position. Sea captains during the Napoleonic wars were often granted a colonelcy as a reward for distinguished actions.
CC
Neither did Anne, but she has honorary military ranks.
https://royalcentral.co.uk/uk/anne-at-70-her-military-appointments-147042/