Having looked at the video, you look to be absolutely bang-to-rights on the contravention itself. You charged into the box with no hesitation at all, and ended up stuck in the box because the car in front stopped and thus made you stop. The contravention is of entering the box and having to stop due to the presence of stationary vehicles, (the car in front).
Of course the PCN can be flawed, and, indeed, Hippocrates has found an error in the text giving the statutory grounds of appeal. However, for this kind of error, the council will never give way, so you'd have to take them to London Tribunals for a decision, and that means the full PCN penalty would be in play. So if you want a punt, take them all the way. Hippocrates may assist you with an appeal.