To be honest, the signage is about what one would expect for such a restriction, I would not call it inadequate at the point where it starts. The problem being more that you were driving on "automatic pilot" as one tends to do on local roads one uses often.
The approach is also via a fairly sharp left turn, giving little time to see the signs and act on them. If you can do some leg work with a camera, go and see if there are any advance warning signs. This would support a case for the signage being inadequate, and thus the council have failed in their duty under Regulation 18 of The Local Authorities’ Traffic Orders (Procedure) (England and Wales) Regulations 1996
https://www.legislation.gov.uk/uksi/1996/2489/regulation/18But the restriction was introduced in March 2025, and here we are in September 6 months later, so whilst you could submit representations saying you were caught out by this new restriction and ask for a goodwill cancellation, don't be surprised if they refuse.
There may be a "technical" appeal, based on Merton mismanagement of the enforcement process. I'm not an expert on these, but it would, of course, mean foregoing the discount as all appeals at London Tribunals are with the full penalty in play. "Technical" appeals are not related to the contravention itself, but to councils not following what the regulations say they must do, the emphasis being on "must".
