THERE IS NOT A SINGLE PROHIBITING SIGN IN THIS PARKING LOT!
No, and there doesn't need to be either, because it is within a Permit Parking Zone. Obviously you missed these signs when you drove along the street and then parked : -
https://maps.app.goo.gl/v9F6nb36Rj3bWt4E8These zones are the invention of the Devil, but entirely legal. Within them, only permit holders can park within the restricted times, and no signs or lines are needed. However, while most of them are 24x7, here, outside the restricted times, there are still bays for permit holders only, so if you'd parked outside the restricted hours you would have been OK.
You could argue that there is confusion because close by where you parked is a permit holder, or pay-by-phone bay, so the absence of a sign where you parked caused you to assume no parking restrictions applied. This zone has been very badly set-up, and you should submit reps on that basis. Don't be upset if they reject them, because councils reject ALL informal reps, and most formal ones. You will only get a fair hearing at London Tribunals, but must risk the full PCN penalty if you go there.
Worth submitting an informal challenge, as they may
****-up their reply