As a general rule, this forum applies potential defences to the facts, we don't try and think of potential facts that might suit a defence. You mention a potential stall, but it isn't clear whether the vehicle
did actually stall, or whether you simply think this would make a more convincing defence (as an aside, it's not immediately clear how a vehicle overtaking might cause a stationary car to stall). You should also consider that the still images are potentially taken from a video, the full version of which may be at UKCPS' disposal. Consider what this may or may not show.
As this may proceed to court action, any argument you advance must be truthful.
It appears based on the evidence that there is only 20 seconds between Photos 1 and 3, used to suggest that the vehicle was parked.
This is, I would suggest, one of the strongest points in your favour. The argument would be that a stop of 20 seconds is insufficient for the driver to read and properly consider any communicated terms.
Are the signs in this area sufficiently clear to clarify which areas are subject to which rules?
You should find this out by getting some pictures. This is likely to also be a fruitful avenue of defence. There's a fair chance the signage also does not communicate a valid contractual offer.