I think that, given a different time and location for the battle, Boudica could win. The Roman victory was one dependant on very favorable terrain that negated the British numerical advantage. With Roman training and equipment, this cost Boudica the battle.
What I'm thinking is that Boudica either moves faster, perhaps do to favorable weather or the ninth legion falling back towards Londinium instead of giving battle, or Seutonius is delayed in his march somehow, and Boudica reaches Londinium either with Seutonius or soon after. This means Seutonius lacks time to evacuate Londinium as he did IRL, as he lacks time before Boudica strikes.
Seutonius was right in deciding that he did not have the forces to hold the city. The terrain advantage he had IRL would not exist, and he would have little time to prepare. His forces would get slaughtered. Even if the ninth legion had fallen back to Londinium and was available to fight, it would still end bad for the Romans.
With Seutonius dead and Roman military power in Britain quashed for the moment, what is Boudica's likely next move? What about the likely Roman response? I want to make a TL out of this, but I don't know what direction to take things in after Seutonius dies.