I disagree, here's what I would send:
Dear Milton Keynes Council,
I challenge liability on the grounds that the alleged contravention did not occur. At the time when the CEO observed my vehicle, I was engaged in loading and assisted boarding. For context, I had stayed in the travelodge hotel and was in the process of leaving, when the CEO observed my car I was making my way back to the vehicle with my elderly father and around 20 KG of luggage. I was accompanying my father to Heathrow airport and his flight was due to take off at 11 am.
While I appreciate why the CEO might have thought I was in contravention, if he had observed the vehicle for only a couple more minutes he would have seen me return to my vehicle and it would have been obvious to him that I was engaged in loading bulky luggage.
In the circumstances the alleged contravention did not occur.
Obviously we don't know at this stage if the traffic order has a loading exemption, but almost all TROs we see have such an exemption so we might as well have a go.