Stamfordman has identified what could be a winning argument at London Tribunals.
Thaousands of people have taken councils and also TfL to the London Tribunals over the years. There is nothing daunting about it, and almost all hearings are by phone or video (MS Teams, I think). No oath swearing, it is all fairly informal. Councils and TfL rarely attend. If you lose you pay the full PCN penalty, but there are no additional costs at all, and if you win, you pay nothing, but Brent will have had the PITA of preparing an evidence pack and also paying the LT adjudication fee.
So it's up to you, pay the discount, or stand your ground at LT, but my view is it is not a slam-dunk win. So for the strongest case presentation, follow Stamfordman's line of approach.