Chariots are very powerful at busting up enemy formations. The Egyptian chariot general is overpowered.

The key is to follow chariots with a cav charge and always charge behind the enemy so the chariots push through.

Seleucid combo of first eles, then scythe chariots, and then cataphracts can bowl over practically any force. I rarely need to move up silvershields to engage as that force already did the trick.

The problem is that the Britons don't have cav. While they didn't have very strong horsemen like the Gauls and Germania, they should at least get vanilla barbarian cav though not noble cav.

In the interim, buy barbarian merc cav any time you can when the Britons and use them carefully as they are worth their weight in gold.