To change formation you simply right click, but before releasing the mouse button you drag it to the right. You can also increase formation witdth by pressing some key on the keyboard. You can press F1 during batle and look it up on the list.
Well, I haven't played the Britons yet, so I can't say much about using their units. But if things are going too fast you can always pause the battle.
Egyptians are one of the easist factions. Numidia to your west is not much of a threat. Seleucids to the north have inferior units at this point of the game so you should be able to beat them easily. At least they use phalanxes alot so battles shouldn't be quite as chaotic - unless you make them that way with your chariots and Desert Cavalry. Speaking of Desert Cavalry, get lots of those. They are excelent at taking out generals, horse archers and they can deal pretty well with chariots as well.
Lord Dragatus, 30th member of BTOOIC, The One Who Killed the Cow.