This is an interesting one. You did enter the box junction without a clear exit (well clear one in front of you anyway!), but started to reverse pretty much immediately back out again. The laws of physics dictate that it's impossible to change from moving forwards to moving backwards without at least a momentary time in between when you are technically stationary, but it's barely perceptible in the footage that you stop at all, and would certainly be 'de minimis' other than for the strictest of adjudicators.
Maybe it's the quality of the footage, but I can't make out any sort of stop line for the traffic lights ahead which cause the stop. The way the box junction is situated is a bit of a trap as traffic will suddenly stop without much warning when the road ahead looks completely clear, so anyone unwary or unfamiliar with the set up is likely to get caught.
While we consider this one, can you post up a copy of your PCN (all sides) just redacting your name and address please? It will enable greater consideration of appeal arguments rather then on the footage alone.