So let's get this right. You parked to go shopping on double-yellow lines. These need no signs, so there were none for you to look out for. Did you not notice these DYLs which mean No Waiting 24x7 ?
The PCN says the restriction is also No Loading, but GSV latest is August 2021, and shows there is no no-loading restriction, so a No Loading restriction may have been brought in since them. So if you are to take this further with us, you'll need to do some legwork and take photos of the street where you parked so we can look for yellow kerb blips and a sign giving the times of the loading restriction. If they are double kerb blips, it is No Loading 24x7.
I get the distinct feeling reading your narrative that you just decided to chance your arm, but lost; true or false ?
Incidentally, you should be able to view the CEO's photos on their website by quoting PCN Number and car reg.