Looking around on GSV, (which is not anywhere near up-to-date), shows signs for part-off-carriageway parking with text below "in marked bays only". This is the sign, if you approach on Oreston Road: -
https://maps.app.goo.gl/MSW4suRhczU9EHQq6and this is the sign if you turn in from Upminster Road North: -
https://maps.app.goo.gl/61TH8jj3Fbv9FR8i8You weren't in a marked bay from what I can see of your photos. GSV does show some marked bays, but whether these are still visible, we can't tell from GSV.
You have a Notice to Owner. Did you not submit any informal reps to the PCN at all ?
If they don't re-offer the discount when rejecting formal reps it is then a total no-brainer to take them to London Tribunals.