Unfortunately you do not have any obvious grounds for appeal. I suggest to try a 'begging' appeal setting out any mitigating circumstances and promising not to be a naughty boy/girl again. Authorities have the ability to allow appeals on this basis but adjudicators don't.
Here are some examples that you can edit to your circumstances:
I believed that my exit was clear ie that there was space to fit my car. However unfortunately I misjudged the length by a few inches. Guessing distance from an acute angle is not easy. I'm only human. I ask that you please cancel the PCN in light of this genuine attempt to exit the box and tiny mistake which caused no obstruction or delay to any other vehicles.
I understand that I should have waited till my exit was clear before entering the box. However at this location the exit is constantly filled with traffic from multiple directions. I would have caused delays to traffic if I did not take the chance and go forward. I thought yellow boxes were supposed to help traffic flow? I'm sorry if I was wrong about that. As this is a first time offence, I caused no delays to other vehicles and based on these mitigating circumstances, I kindly ask that you allow this appeal and cancel the PCN. Thank you very much.