Thank you both, that makes sense.
I am very surprised there hasn't been a technological solution to this problem. Something like the congestion zone where if you are a local resident, you register your vehicles with the council, and then at the entrance to these areas you have cameras that either let you through (if your vehicle is registered as local), or you are fined. This would also work for Manor Mount (no access from Honor Oak Road) as well, where currently for me to go to Sainsburys I have to take a very environmentally unfriendly route (lots of sitting in the red bit, which gets very long these days) vs nipping straight down Manor Mount (which is understandably closed or people would use it to get to the South Circular via Davids Road).
The technical implementation of this would be simple, but wouldn't make a lot of revenue once it was in place, which I suspect rules it out as a solution.