Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Suetonius was doing what any sensible general would do, which is to select a seat for war that was easily defended ... so he was not wasting his resources attempting to defend the indefensible.

It depends what is meant by 'seat of war' - a rallying point for troops or a place to make a stand.  Tacitus implies the latter when he says that Suetonius looked at the paucity of his troops (circumspecta infrequentia militis) and decided that it could not be London.

He then spent the next months, perhaps a whole year, preparing for war ... that is why a general selects a "seat for war" ... to prepare.

If he did, his time was not well spent, if he could only raise one legion, which was probably with him already, the veterans of another and the nearest auxiliaries, of which there were likely to have been few south of the Thames

Then, when he was ready, at a time of his choosing, he drew Boudica into battle at a site of his choosing.

How was this achieved?  Did he attempt to lure her south of the Thames, an invitation she would most likely have refused?  Or did he cross the Thames looking for a likely battlefield and vulnerable to attack until he found one?  It must be remembered that Dio says that he was forced to give battle against his better judgment because he was running short of food and the enemy was pressing upon him.  In saying that he decided to cease delay and give battle, Tacitus is more than likely making a virtue of necessity.

Why not bring the legions down from north Wales, draw in reinforcements from the Continent and march out when he was sufficiently reinforced?

Did he need to leave these other areas undefended?

Drawing reinforcements from the Continent would not have left it undefended.  The troops in north Wales were campaigning outside the defended area of the province, so calling them down would not have left anything undefended.  It would have meant abandoning his conquests but he did that anyway when he consolidated his forces for his campaign of reprisals, leaving Agricola to reconquer Mona when he became governor.

Unlike the ninth who rushed into battle without preparation, we are told Suetonius did not rush into battle, so he spent his time preparing for battle. That is why a much smaller force was able to defeat a larger one.

Have you read Adrian Goldworthy's paper 'Instinctive Genius'?  He makes the point that warfare is not conducted in the same way in all periods and in all cultures.  He demonstrates that the Roman method of tackling uprisings was to attack immediately with the forces available, even if they were under-prepared and inadequately supplied, with the object of intimidating the rebels with a rapid show of force.  Sometimes it worked, sometimes it didn't.  If that was what Petilius Cerialis was attempting, his failure did his reputation and career no harm because, despite Tacitus' criticism of his temeritas, he had done what was expected of him.

Suetonius was more cautious and the rebellion had passed its initial stage before he could intervene but he would nevertheless have been aware of the expectation that he would engage the enemy as soon as possible, before the rebellion gained further momentum.  If he had retreated south of the Thames, this would have been seen as a sign of weakness which Boudica would not have hesitated to exploit.  The tribes north of the river would have felt themselves abandoned by Rome and those wavering in their loyalty and even those sympathetic to the Romans would have thrown in their lot with the rebels to protect themselves from continued or anticipated depredations.  Furthermore, Suetonius would have had to keep track of the progress of the rebellion and he could only do that by keeping himself within scouting distance of the rebel forces, not by sheltering south of the Thames.