With the Franks and Celts, the Britons are one of the civilizations belonging to the Western Europe region. Its major strength is in its archeries 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 Bombarb Cannon(Turkish unique technology increases the range to exceed the Briton Elite Longbowmen) and ofcourse Trebuchets.
The Longbowmen are great for defending since most units won't make it to the Longbowmen, when attacking remember to take a few bombard cannons or trebuchets for castles and towers, although the Longbowmen outrange most defensive structures (Apart from the Teutonic Castle after Crennelations is researched) they do take quite a while to destroy the structures since they do pierce damage. You might also find it usefull to move a monk or two with the army for healing. ;)