Your post shows the peril in using satnav in London ! It's got to the situation now, where I call PCNs like yours a "satnav-assisted PCN". Satnavs are never up-to-date, although looking at Google Street View here is fairly up-to-date, and it shows the signs you passed. So maybe you need to check your Google Maps installation for updates.
As you have found, we've already got a case for this location.
I assume you drove up Vauxhall Street then turned left, (the right turn is a timed restriction and also shown as a dead-end: -
https://maps.app.goo.gl/GfHf6JQo9ZCi5rYt6then as you drive along you come to the Clayton Street junction approach which has a fairly prominent mandatory "Turn Left" sign. Ahead are the two restriction signs that you drove past. Essentially, it looks like this side of the Oval has been restricted in order to stop rat-running like yours, and Harleyford Road is now the only through road round the Oval.
https://maps.app.goo.gl/5UMvSHhPYj78XhLK9It's difficult to think of a robust appeal here because all the signage is in position and would seem to meet the test of "adequacy".
However, there may be a technica; appeal based on council mismanagement of the enforcement process, so wait a bit to see what is suggested, but if you're minded to pay the discount, don't miss the deadline for that. Technical appeals have nothing to do with the actual alleged contravention, and concentrate on how the council are carrying out their mandatory enforcement duties. They have to obey the law same as you.
