Calling all armchair generals! Boudica's Last Stand.

[Renatus]

The problem with the south-east route is that taking it would have separated Paulinus even further from any hope of reinforcement.

I'm not sure whether there was a bridge at London in AD61, or it was still a ferry crossing. Either way, Paulinus would surely have destroyed the crossing point while evacuating the city, leaving the Thames as a major obstacle. For the Iceni to follow him - considering they were moving with a large number of carts and a multitude of noncombatents, they would have to travel twenty miles west along the Portway to the bridge (or ford, perhaps) at Staines, cross over and double back around. Why do that, when the whole Thames valley lay open and undefended before them?

Paulinus could have gone south across the river, as Steven suggested above, to convey the civilian refugees to safety - but Tacitus is clear that they had to keep up with his march, which implies there was no major river obstacle between them and the enemy.

I agree. If Paulinus had gone south, he would have destroyed any bridge behind him (even, perhaps, detaching a small force to destroy any other nearby bridge that the rebels might use) and put the river between him and the enemy. They almost certainly would not have troubled to travel twenty miles in order to double back and follow him. The clear implication that both forces were in the same side of the Thames almost guarantees that Paulinus did not go south.