Plus they can get some super-brilliant governers and tradesmen.

My own starting tactics were similar to ones proposed here - I sent my diplomat across the channel immediately to start his long walk around the world getting trade agreements and cheekily bartering map information here and there, then instantly disbanded my single ship so I didn't have to pay the huge upkeep, then ramped up the taxes in Camulosadae. Don't go to war with the rebels until you're absolutely ready, since you immediately take quite a hit in your finances by going that route.

A short while later I waited until the last possible moment to gather a force to go north and strike Ratae (since upkeep is the big killer in the Casse early-game) so that most of my expenditure was a one-turn hit that I could regenerate slowly once Ratae was captured.

The early-game sieges are a piece of cake. A core of slingers is key, and the AI is stupid enough to keep making forays out into the range of them, but since I disbanded the army immediately after I was victorious at Ratae, casualties didn't really matter, I suppose.

Once you have two provinces, your profits should be rather small, but steady. Through this you should be able to concentrate on building structures to increase your trade, farming and profits in any way possible. You will be in the red for quite a bit of the first few turns, and will spend alot of time doing nothing but hitting the end turn button.

Once you get the third province (using the last minute recruitment scheme mentioned earlier) you should have enough money to feasibly maintain at least a half-decent force ready for your next move. After that, Britannia becomes your oyster.

In my campaign (H/H) its 245bc, I have all of England and Wales, I'm making 4-5k profit per turn, and have 25k in the bank at the moment, and a nice big group of family members.