In every game of RTW I ever played, Britannia wound up at war with Germania, Gaul, or both at once. They usually conquered whatever the Julii failed to take. Sometimes they'd go after Germania first, and sometimes they'd go straight after Gaul, but they'd always eventually get around to the other. I never tried modding them so they didn't start with Samarobriva, which I understand tends to keep them on their island. Perhaps that's why RTR removed them as a faction.