There were female warriors in some Celtic armies, and while they weren't really a miniscule number of exceptions, they also didn't usually ever occur in a great enough number to actually have entire units of them.

Boudicca actually wasn't a warrior, she was more or less pushed to fight out of anger. Her 'army' mostly consisted of non-professional peasants. When she was defeated, at the battle, the families (including wives, the important part there) of the men fighting turned up at the battle to watch it, not fight in it. So, while female warriors did exist, until we can have a unit with multiple, random models in it (and have a female model that would show up rarely), it would be a big anachronism to have a unit of female soldiers in the armies of the Britons.