First I would pinch myself, trying to make myself come back through the looking glass from 1998, as that was the last time I recall that DL sold First Class across the Atlantic!
Second, as it seems that you were in the bump game, you blew your big chance as you would have been entitled to a lot more for the second bump than the first. Personally I think that the $200 was too little and you were bought off too cheap. That was your fault, not DL's.
You can try complaining, but your real opportunity was at the time. Personally I think that DL had every right to sell the seat for $6k, you just should have been more proactive at the time.