Queen Boudicca was the wife of the king of the Iceni tribe that lived in the East Midlands. When here husband died, she automatically became head of the tribe according to Celtic custom. However, the Romans did not recognise women as having any leadership rule.
Eventually, while the Roman commander was in the north, Boudicca led a revolt, destroying towns as she and her people moved south to London.
She was able to overwhelm the Romans who were left in London and set fire to the city, but in the long run she could not be a match for the Roman army and the Iceni were defeated. Queen Boudicca killed herself instead of being taken prisoner.
Boudicca is held up asa traditional symbol of British courage against an oppressive enemy. The symbol of Britannia is used the same way.