OK, so looking at your photos, the vehicle was mostly blocking the footway, hence the PCN. Parking off-carriageway has been an offence in London since the 70s, and needs to signs or lines. If you'd parked on the carriageway to unload you'd have been OK.
There may be a "technical" appeal argument, so wait and see what the others say. We always recommend submitting representations, even if it looks as if you are totally bang-to-rights, as they may shoot themselves in the foot when replying, and also it give us time to make further investigations. They will re-offer the discount if you submit reps within the discount period.