Okay. I made a mistake. Postumus wasn't the legate of the II Augusta, he was the camp prefect (Top Centurion).
Boudicca ended her days by poison; while Poenius Postumus, camp-prefect of the second legion, informed of the exploits of the men of the fourteenth and twentieth, and conscious that he had cheated his own corps of a share in the honours and had violated the rules of the service by ignoring the orders of his commander, ran his sword through his body. Annals (Tacitus)
Nero sent reinforcements from Germania, which accounts for the mentions of Gemina.
The whole army was now concentrated and kept under canvas, with a view to finishing what was left of the campaign. Its strength was increased by the Caesar, who sent over from Germany two thousand legionaries, eight cohorts of auxiliaries, and a thousand cavalry. Their advent allowed the gaps in the ninth legion to be filled with regular troops; the allied foot and horse were stationed in new winter quarters; and the tribes which had shown themselves dubious or disaffected were harried with fire and sword. Annals (Tacitus)
The commentary from Bill Thayer is as follows:
The probable course of events seems to have been roughly this:— Suetonius hurried ahead with his light troops, while the fourteenth legion and part of the twentieth followed by forced marches: the second had been summoned to join him, probably at Wroxeter (Viroconium), but its commander Poenius Postumus refused to leave his own front defenceless against the Silures of S. Wales. At London, the situation was found to be desperate, with the rebels in overwhelming force and the ninth legion virtually exterminated. Suetonius, therefore, fell back along the Watling Street until he met the legionaries, was forced to an engagement "somewhere in the Midlands," and only survived through being allowed to choose his own ground. (Bill Thayer)
This implies the legate of the second legion was not present with his men. This is not explained.