Londinium was without defenses. The population had time to leave and even a few places to go. For some reason there are always people who refuse. They died. When the 2nd Augusta ( I think they were the first legion to return) saw what had been done, they did their best to eradicate the Iceni. The Romans were big on making examples.

Not to say that Boudicca was entirely in the wrong, not by any means. When you commit atrocities, you had better win the war. Especially against the Romans. Remember the Servile Wars, 32,000 curcified.