With the Franks and Celts, the Britons are one of the civilizations belonging to the Western Europe region. Its major strength is in its archers and long-range military units. Its unique unit, the British Longbowman, is the unit with the longest firing range in the game apart from the Turkish Bombard Cannon (Turkish unique technology increases the range to exceed the Briton Elite Longbowmen) and of course Trebuchets.
The Longbowmen are great for defending, since they might be able to kill enemy melee units even before they are reached. Trebuchets for castles and towers, although the Longbowmen outrange most defensive structures (apart from the Teutonic Castle after Crenellations is researched). They do take quite a while to destroy the structures since they do piercing damage. You might also find it useful to move a Monk or two with the army for healing.
British archery ranges train troops 20% faster and British town centers cost 50% less. British Archers have +1 range in the Castle Age and +1 range in the Imperial Age (+2 total). British shepherds work 25% faster.