I would imagine a Belgic faction would be a confederacy of sorts. To my mind there are three strong candidates for a Belgic faction; 1. The Suessiones who Caesar informs us controlled areas of Gaul and Britain in years preceding the Gallic wars. 2. The Bellovaci who by the 1st Century BC were the most powerful of the Belgae (my money is on them). 3. The Atrebates as they are the only Belgic tribe we know for certain migrated to Britain.