I dread to think of how many tickets Royal Mail vans (crown vehicles)get every day
If they are collecting/delivering mail then they have an exemption from Parking Regulations. If, however, they are in the local cafe having breakfast/lunch/dinner, then they haven't.
The fact that Royal Mail cannot be bothered to even write a generic letter of appeal is of course their decision, but I strongly suspect that the CEOs in question only issue the tickets on the assumption that Royal Mail won't actually have to pay them, but it contributes to their (alleged) Target of XX PCNs per shift.