If Boudicca had successfully driven the Romans out of Britain, they would have come back and reclaimed the territory within a few years, if only to save face. The Romans made a big deal about avenging defeats, which is why Germanicus launched his campaigns in Germania after Teutonburg Forest and why they launched so many campaigns against the Parthians after Carrahe.
You are right on the mark good sir, if Queen Boudiccia's mob of allegedly 200,000 barbarians had managed to overwhelm the Romans under Governor Paulinus through their overwhelming numerical superiority, the surviving Romans would have tried to evacuate rather than being wiped out.But how many would succeed?
Not many I am afraid. The Classis Britannica would obviously do its best to save soldiers and Roman citizens, but IMCO transport would be far too inadequate to that task. Boudicca's warbands would have enjoyed successes similar to the sacks of Camulodunum, Londinum and Verulaminium when they went for Glevum, Calleva Atrebatum, and Eburacum.
Nero I think
would have panicked and ordered the province abandoned. That would have been a HUGE blow to his credibility and legitimacy and might have brought him down prior to his 69 AD fall in OTL.
Whoever emerged from the Year of the "n" Emperors would have felt obliged to reconquer or at the very least invade to harshly punish the Britons as Tiberius and Germanicus did in Germany after the Varan Disaster.
If Vespasian wins the civil war as in OTL it is my considered opinion that he would have made sure Britannia was reconquered or at the very least severely punished and reduced to client status. He would have had scant choice given how Romans historically treated defeats..."peace can only come after a Roman victory." Maybe his son Titus would have been supported (unlike Agricola by Domitian) with enough Legions to enable the complete conquest of Britannia-Caledonia-Hibernia.
"Titus Flavius Britannicus Maximus" has a nice ring to it!
Hero of Canton