The Britons is a land civilization, meaning it excels in battle on land. The Britons' specialty is archer power. They pump archers out faster than anybody. Their unique unit (an archer) has the most range of any archers. The British navy isn't shabby either, lacking only the Cannon Galleons. On a downside, the British don't have any gunpowder units or structures, and may not have the adaquet firepower. They don't have Imperial siege weaponry: Bombard Cannons, Siege Rams, and siege Onagers. The Britons don't have the famed Paladin to round off their cavalry either.
The Longbowman is definately a vital part of the British military. With more range than any other archer, the Longbowman should make up a sinificant amount of your British army. It is advantageous to use the Longbowman in medium numbers, on high ground with protection, and in flank formation. Longbowman should engage in only long ranged combat. All archers must avoid siege weaponry and should should focus on doing damage to the enemy's slow land units. When your Longbowman are threatened, move in your cavalry to slaughter the offender. Build the Longbowman in large numbers, as it is relatively cheap.
The British tech: Yeoman, increases the archers' range by one and gives towers +2 attack. This technology costs 175W and 450G. Needless to say, this tech is very important to research since the Britons depend so heavily on archers. You should keep in mind that the unique tech of any unit is always a must have.