This is an easy appeal, as the video shows no contravention. Send this for now:
Dear Transport for London,
I challenge liability for PCN GX10193723 on the basis that the alleged contravention did not occur. This is because while my vehicle did stop within the yellow box markings, that stop was not caused by the presence of stationary vehicles, as the video clearly shows that there is ample space ahead of my vehicle outside of the box.
It follows that the PCN must be cancelled.
Yours faithfully,
Send that online and keep a screenshot of the confirmation screen, I'll be happy to represent you at the tribunal if they reject.