I think maybe this post has been forgotten?
You're lucky it didn't get towed, because you'd have got a massive bill for tow and storage. It's very unwise to leave cars unattended in London for long period, (18 days in your case), because parking suspensions are very common.
I suggest you submit an informal challenge stating that when you parked the car on 1st April there was no sign for the bay. Your GSV link shows there was a sign there in July 2024, not all that long ago. So in your reps you should also ask them when the new sign was put on the pole.