Here's a campaign I found myself starting as the Britons and decided to share.
You control the British Isles, Northwestern Gaul and most of Germania. You have tons of cash in the treasury, plenty of family members and a couple of armies in the East. The next part of the game should not pose any problems; Germania is all but spent. It's after that that things will get interesting...
(Sorry, I don't remember exactly what year this is in.)