God, what tosh ! However, they don't say you cannot load in these bays, but must establish a exemption. The CEO saw you at the time for heavens sake, and you told him you were loading !!
There is no requirement in law or the regulations for loading to be attended all the time. But of course, this is the usual Fob-Off letter we see so often. If it were me, I'd be taking them to London Tribunals, if the TMO permits loading in these bays for disabled permit holders. Of course if it doesn't, pay the discount.