Yo hi all Brittania fans! I'm playing a short campaign on M/M v1.2 with this cool faction and must say Brits totaly rock! Yeah man their barbarian-fanatical units (Woad Warriors , Head Hurlers , Druids (their chanting is so COOL! )) and British Chariots make all the foes piss their pants and rout very quickly, especially if you attack before war cry.
So here's what I've learned from playing this campaign. Firstly took everything I had from Britannia Island to mainland, left only a minimal garnison (1peasants) in each town and attacked Gaul's settlement Alesia. As guys here advised I signed treaty with Germans to prevent war on two fronts (later those bastards attacked me still but I've wiped out their full stack army ). So once I was at war with Gaul I didn't stop and later Gaul faction only had 2 settlements in Northern Italy and one in the center of Spain. Now it's about 240bc in my short campaign (cause you can get all this faction's units in a short period, not like e.g. romans), I have 13 provinces (2 remaining and the destruction of Gaul for victory), (also I kicked some German ass before that.)
What about economy? First priority would be ports, roads, farm upgrades (if the fertility is high) then comes trade increasing buildings (trader, market ...)
At the beginning I only build military buildings in few town where they where needed the most, cause Brits' economy is poor at the start of the game, you need to improve it. So focus on economy at the beginning
Must mention that it is important also to build watchtowers. They are very cool for monitoring bandits' and especially enemy movement. Find a suitable location for them (mostly on a hill or road). I find forts also very useful in chokepoints (like river crossings, mountain pass). I keep in them a garnison of about 4 units (1 ranged, 1 cavalry, 2 infantry) to destroy bandits and also to defend borders.
What about army composition? Well my most balanced army as Brits would be like this: 2-4 brit light or heavy chariots on the wings of army (although I like archer chariots more) - this is the best unit of Britannia IMO, 2 ranged foot units (slingers or later head hurlers) place them in front of the army, 2 barbcavmerc if you can afford it (heavy cav is the drawback in Brits army), few units of wardogs, the rest is infantry: warband (later swordsmen and chosen swordsmen) as the main line, druids just behind them to support fighters with chanting, on the infantry wings there should be some units of Woad Warriors or Swodsmen for flanking operations, and of course the army should have a decent general. I place him behind main force in the center. Remember always to use the war cry as this improves morale and gives bonuses for attack. So that's about all I've learned while playing this wonderful faction BRITANNIA! Yeah WE SHALLlll RUUUULE! FEARRRRR USSSS! Good luck everyone!