You could appeal on the basis that you were unable to leave due to traffic congestion - they will probably reject, but it puts them on notice. Make sure you appeal as keeper only, and refer to the driver in the 3rd person. The PCN is missing the invitation to pay or name the driver which is a point for POPLA or the small claims court if it goes that far. The signage may be a point for POPLA as well - it looks like you parked in the small bay on the right of the entrance - if there are no signs as you enter, and turn into this area, then you could argue for inadequate signage as well - but this will not hold much water if you drove around the carpark 3 times first, past all the other signs, and then entered the store passing the sign at the door (there is one shown on Street View, but it refers to different terms)!
This appears to be an M&S owned car park, so, assuming you were a customer, you should also get on to them and ask them to get it cancelled.
