What if instead of stop sign it was a much slower cyclist or pedestrian ahead?
Would you have crashed into them?
Would you have quickly and with insufficient notice to the drafters, swerved left/right to avoid them, with the two cylists following you instead crashing into them?

I know the penalty for running a T-intersection stop across the top seems steep, but given other circumstances the lack of attention by you and the following cyclists (they should not be only watching the wheel of the rider ahead) could have resulted in far worse consequences and not just to you.

That said I do hope you can reduce the fine, but consider the lesson as well. I also believe you are a safety concious rider as you say and you also note you were lucky, so I have no doubt the lesson has already started to hit hard.

Al