Calling all armchair generals! Boudica's Last Stand.

[Nathan Ross]

if there was a "cavalry dash" it was a mission assigned to a younger, fitter and more expendable Tribune.  My own opinion is that he marched south with his entire force. 

Yes, I agree with most of that.

The army for the Anglesey campaign would have comprised several legions and many auxiliaries, though, and I think Suetonius would have wanted to leave the majority of them to hold the territory he had captured (albeit he had to give up his conquests eventually!).

My guess would be that he marched south with an expeditionary force of the maximum size he could muster quickly and that he knew he could keep supplied on a rapid march. A single legion at campaign strength (c.4000 men) plus a couple of cavalry alae and maybe a couple of aux cohorts, perhaps - c.5-6K men?

Suetonius would have expected to meet up with Cerialis's detachment (perhaps 3-4K?) on the road, and probably estimated that their combined forces would be enough to crush the revolt. As it happened, Cerialis marched on ahead and was routed, and the rebellion proved to be much bigger that Suetonius had anticipated.

going west seems to me logical as any reinforcement from Legio II and its auxiliaries would be most welcome.

Reinforcements from Usk (Legio XX) or Exeter (Legio II) would both have had to pass through Cirencester on the Fosseway. From there they could swing south-east via Silchester to London or north-east via the fort at Alchester to St Albans. So either Silchester or St Albans would make a good rendezvous point.

St Albans, though, is closer to the larger reinforcement group coming down from Wales, and is on the line of communication route from Wroxeter. It's also closer to upland terrain in the Chilterns that matches the battle site Tacitus describes. Tacitus mentions St Albans and does not mention Silchester.

For those three reasons, I continue to find St Albans, and the 'north west route', a more likely option.