From a post to DAVIDWEBER.NET forums on 6/20/2011

Admiralty chain-of-command

    The only member of the Admiralty who can give a direct order to a fleet commander (or to any other officer of the RMN outside his/her own bureau's chain of command) is the First Space lord, who can give orders to any serving officer of the RMN or the Royal Marine Corps. The First Lord (as opposed to the First Space Lord, is a civilian office, although it is often held by a serving or retired officer of the Navy. There are chains of command through which the Space Lord is expected (indeed, even legally required) to operate under any but the most extraordinary of conditions, but his authority is paramount. Except, of course, that he cannot give direct tactical [orders] to a unit commander or supersede the authority of the chain of command within a unit. That is, he could order a ship in which he was a passenger to attack the enemy; having done so, however, he could not interfere with that ship's CO or the CO's decisions in the execution of those orders. In fact, he couldn't legally tactically override the senior PFC of a Marine squad if that PFC were the senior surviving Marine present from his unit's original TOE.