Fair enough to have that opinion, but what did the flashing achieve? Who was he being inconsiderate to,
bearing in mind the motorway was empty? You had a whole outside lane to use so he was hardly in your way.
Sorry if I come over as a grumpy old git, but I get a bit angry at the 'lane fascists' on motorways who think it is a useful excercise to flash those who they think are blocking their important progress, or who cut in quickly and dangerously after the overtake to show their displeasure. If I'm doing 70 in lanes 1 or 2 and you want to overtake at an illegal speed you should do it safely and considerately. Anything else is just a punishment pass that you no doubt have suffered when cycling.
Rant over.