i'm leaning towards casse myself, however casse campaigns tend to take a long time to get going as you start off neutral to your surrounding independent tribes and as soon as oyu go to war with one, you go to war with them all.

this leads to really aggressive AI from the rest of the tribes on the isles of tin, which means you are now in financial hardships.

a period of maybe 5-6 years might be required purely building up the infrastructure in an attempt to be able to support any armies you are recruiting for the subjugation of the rest of britain