Originally Posted by
Derc
Germanic factions in BI are meant to pillage. The Romans living nearby have very rich but unstable cities, which you should take advantage of. Not only will it weaken them, it'll also ensure that you have enough money to recruit new units/mercs.
Holding the mainland may not be worth it at the beginning of the campaign. A horde will swoop in sooner or later, forcing you to spend a fortune on defending these lands.
Roman Britain is a worthy place to hold for the Saxons, just as it was in real life. The Celts up in the north can be annoying, but they are usually quickly dealt with. When you established a new home in Britain, you can scout out the mainland and easily conquer whatever the other hordes left behind.