These days I thank every driver who notices me at roundabouts and waits like they're supposed to, or cars who slow down behind me when the lane is too narrow to pass, etc. I thanks lots of drivers every day. The idea is to generate good returns; if the cager notices you as a friendly rider who has the courtesy to thank them for obeying traffic rules, then you are likely to get good treatment next time round from the same driver. I would have waved a thanks. Flipping the bird has probably ruined your chances. You will likely have an ongoing feud with this guy.