I've played quite a bit more, really the best way to deal with the German spearman is to pelt them with slingers and light chariots, luckily the AI is dumb so they tend to send one unit at a time. If it was a human vs. human situation of Britannia vs. Germania I dont see it even being a close contest.

More thoughts: head hurlers' use is limited. They need to be a lot closer to loose their missles than slingers, unlike javelins the head hurlers stink at lobbing heads over units. Like Roman javeliners if charged they dont seem to have time to get a shot off before they run away. If you manage to get them to work perfectly they can be devastating but 90% of the time they're fairly useless and often do something stupid and kill a ton of your own men.

I have more respect for light chariots than I used to, they can be used to take out enemy generals, and if charged by lighter cavalry while in the cambrian circle they usually take it out, they also make the enemy concentrate on them which open the enemy up to a charge from your other units. They become less useful when the Romans start bringing the heavier cavalry to battle though. Also they have an annoying tendency to act like idiots and charge randomly into the enemys lines and then just sit there waiting to get killed.

Heavy chariots are not a match for the Roman heavy cavalry, they get bogged down when they hit the heavy cavalry and get killed rather quickly. Against lighter cavalry the heavy chariots are pretty devastating.

Chosen swordsman one on one are somewhere between a Early Legionary Cohort and a Legionary Cohort in overall power. The cohorts dont attack as well but their armor is much better. Obviously Britannia's best unit.

Well thats about it, the chariots and head hurlers need some tweaking. On normal difficulty the game is definitely winnable with Britannia, I'd take out Germania first and weaken Gaul leaving a buffer between Gaul and Rome and then start going after the Romans and the Spanish peninsula.