If have played my campaign a bit farther:
A: The Britains have to die, period.
They will attack you no matter what you do.
Be sure to take the first strike.
Use Bordesholm on low taxes to get a naval base and assume supremacy in the noprhern sea.
You dont have to invade Britain, just crush their navy.
You could even try to make peace after their navy is done for.
In the east some ok rebel provinces (vicus Gothi is nice) are yours to take, expand until Pripet and Tribus Gepide.
Do not engage another power on the "eastern front".
Try to take out Gallia after the Brits are contained, they have a medeterran acces and can get filthly rich. Try to get to good relations with rome too, you can beat them but you should not do it right now.