Chariots were outdated by the time of EB: the only reason the Britons are still using them is because they lack proper heavy cavalry. To make things worse, the engine cannot properly model chariot engagements, so they are even weaker than they should be. You should keep them out of melee combat, and watch out for missile troops (especially javelineers, which get a bonus against chariots) and charging cavalry. That said, chariots have four uses:

1) Breaking up infantry formations: order your chariots to charge through an enemy infantry formation and follow up by heavy infantry: this will play havoc with enemy combat efficiency.

2) Breaking up cavalry formations: chariots are one of the few units that can effectively counter charge heavy cavalry. Don't get stuck in a melee, though, or allow your chariots to be charged when they are bogged down.

3) Missile support: chariots are skirmisher cavalry that carry loads of javelins. Use them as such.

4) Morale support: Briton chariots improve the morale of nearby allies, while scaring nearby enemies. Combined with javelins and the ability to break through enemy lines, this can cause a massive morale penalty, allowing you to quickly rout all but the best units.

The main advice about chariots is that they should be kept moving. A mobile chariot is deadly, an immobile one is merely dead.