While no doubt a distinguished gent, can you really describe MacMillan as "a fine SOS" when he was in post for less than a year, Leon? One might also add to your list Lt Col Duncan Sandys, who in the 1950's declared the days of the manned fighter to be over and by the by shut down much of the UK aerospace industry. And perhaps the name of Brigadier John Profumo rings a bell, though not for the right reasons?
The point is, most of the current candidates are well into their 50's or older. How relevant is what they were doing as 20 year olds? I suggest that a military link on the CV is no guarantee they'll be a great SoS thirty years later.