See
here to edit your file to make them available, thought they can be unlocked by campaign without editing.
Britannia is great fun. Your generals are mounted on chariots with 5 hitpoints. Unless the enemy manages to stop you moving, these guys shouldn't go down. Chariot archers are good and all but immune to the cavalry that can chase them down. The lack of cavalry is problematic sometimes when trying to chase down routers after a battle, but you can always move your chariots in front of the enemy so that they will stop running in that direction and change direction allowing your infantry to catch up and kill some more.
Infantry-wise, Chosen Swords and Warbands are solid and their Woads are fairly strong for an infantry that can attempt to act as a cavalry-replacement. They're a bit weak in defence though, so probably best suited to flanking work. Druids are handy, especially if you've got no general for your army, but I don't use them often.
Ranged units are interesting. Slingers are okay, though the units they fare best against, chariots, aren't to be seen in any nearby faction. Slinger attack isn't likely to cause many casualties in most cases, though as always ranged units are useful for drawing the enemy in to traps etc. The best unit in the roster IMO are the Head Hurlers. 17 attack armour-piercing missiles. Sucks for the Romans. A couple of volleys from these fellas and the hardest Urban Cohort will be reduced to half its size and take a hefty morale hit too.
Warhounds are good for chasing those annoying Scythian horse archer units, though you could just as easily use chariots, but in their absence the dogs do a good job too.
Their position is terrific in campaign. The tiny sea trade routes generate good money and the enemies they take on in the early game are Gaul (a pushover with your chariots because they can't deal with them at all) and Germania (a bit harder because of their spears). Germania is in general very weak against missiles though, so if you get them early enough they wont have access to archers (because they have to wait until their top tier for Chosen Archers) to be able to deal with your chariot archers so you can have some of these units circle the enemy without worrying about cav retaliation as they will rip them up easily. By the time Britannia reaches the Romans you should be able to have plenty of Head Hurlers in your armies to ruin all the enemy legions before your units engage and rout them.