OK, so you were loading pre-ordered goods from B & Q and the bay is at the back of pedestrianised St Anns Road on which B & Q lies. Your car was parked here, all spaces nearer St Annes Road being occupied : -
https://maps.app.goo.gl/Z1XiqErN8p5iBz4i9So you should submit representations that you were engaged in loading of pre-ordered goods bought from B & Q. It is important to emphasise "pre-ordered' because just going shopping and leaving one's car in a loading bay fails the test of "loading". You will need to state that on returning to your car, you spoke to the CEO and explained that you had collected goods and were now loading them into your car. DOn't be put-off by council tosh that usually says "loading must be continuous and the vehicle attended". This is total tosh and they know it. HOwever, they will automatically send you a FOb-Off letter because they know that >95% of people then cough-up to get the discount. You should also put in that the CEO told you the bay was for commercial vehicles, which is also complete tosh. This CEO needs a training refresher !
There may also be a 'technical' appeal argument based on Harrow mismanagement of the enforcement process, incl. fatal errors in their web pages for paying and submitting reps. Our Hippocrates is something of an expert in these and has a few, (nay, a lot of !!), council scalps to show for it.