To be honest, I can't see anything wrong with the PCN, and the sign is clear albeit on the off-side of the direction of travel, but seems clear enough, although being on the off-side, it could be obscured by high-sided vehicles, but the photos show no traffic like this.
Discount looks the best option IMHO. You could have a punt at London Tribunals on the description of the statutory grounds of "driven without your consent" that implies it's restricted to the car being stolen, because there's no restriction like this in the regulations, but the full PCN penalty would be in play.