Looking at your GSV link, you can see that there were white bay markings, but they are virtually all worn out, so as you say, you went to a shop to ask about the bay. So what did the shop tell you ? Funnily enough, whilst the bay markings are almost totally absent, the "Loading Only" text on the carriageway is still clear.
I suggest you submit representations on the basis of the contravention did not occur, because the bay markings worn out so requested info from one of the shops. They told you it one bay and all for goods vehicles loading only, so you immediately went back to your car and drove off.
BTW, the postal PCN is lawful under the Traffic Management Act 2004, which introduced the power to serve a postal PCN if the driver of the vehicle drove off before the CEO could serve a PCN at the roadside.
