Calling all armchair generals! Boudica's Last Stand.

[Renatus]

St Albans would perhaps make a rather better 'base of operations' anyway, as it was at the junction of two supply roads, marginally closer to his approaching reinforcements from Wroxeter, and closer to the high ground of the Chilterns.

You have made the point that sedes bello can have two meanings and, although I hold with the Michael Grant interpretation so far as London is concerned, I accept that the other meaning could well apply to other locations.  Suetonius would have needed a rallying point for his reinforcements and, if any one wanted to call that a sedes bello, I would not necessarily quarrel with that.  Having said that, it need not be a fixed headquarters.  The headquarters of a campaigning army would be wherever the commander was at the time.