Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Alternatively, I could see him resting his troops at St Albans after a direct march south, perhaps, and pushing on the last 21 miles to London with a light vanguard - perhaps to meet with Decianus Catus, if he didn't know he'd already fled, or to assess the possibilities of fighting a battle there once his full column arrived. That would not take him too far from his command, I would say.

Unfortunately, I do not see that explaining 'medios inter hostes'.

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I see leaving all the veterans of 16 years service to guard legionary fortress as robbing the army of experienced men who could have a positive influence on new recruits, while on campaign. Allow me share my research.

Your breakdown would seem to work administratively but the situation on campaign could be different.  If there were men in the unit who were not expected to do anything unless it came under attack, for most, if not all, of the time they would be just dead wood.  Those with 12 to 15 years' service would still have much to teach new recruits.  I don't think that an extra five years would make much difference.  Moreover, if a legion moved out of its fortress temporarily, someone would surely have to be left to look after it and who better than those no longer needed for active service.

Turning to Tacitus' account, we are told of the presence at the final battle of only one complete legion and the veterans of another.  We are told also of another legion apparently commanded inexplicably by its praefectus castrorum, the third in command, with the legionary legate and tribunus laticlavius nowhere to be seen.  I have put forward what I believe to be a reasonably satisfactory explanation of this conundrum.  There may be other scenarios that work equally as well or better but I would like to see them and consider whether they might cause me to revise my opinion.