You could quote the highway code: “Parking Your vehicle will be considered “parked” if you stop longer than necessary for loading/unloading or boarding/disembarking.” If you didn’t stop longer than necessary then you were not “parked” but “stopped” and they can’t possibly charge you for parking.
Thanks. Good idea.
Current car: Mini Countryman EV in very shiny black!
Previous car: MG HS Exclusive in nice shiny black.