Basically your lack of knowledge on the signing of parking bays and their restrictions has caused you to get a PCN, so it's difficult to think of any credible appeal based on the contravention itself.
However, I'm struggling with GSV, because you say you parked in a bay which was residents only all days, but saw a sign opposite that gave Mon-Sat restrictions only. Yet when I look at the location, I see the sign in this bay which is all days 8.30am to 8.30. WIth no days specified it means all days: -
https://maps.app.goo.gl/iEAJfxmPxEvd16Uh6Then, going across to the bay on the other side of the street there's a ruddy great van blocking the view of the sign ! So looking a the previous GSV view (Nov 2022), the sign says residents only Mon-Sun, (i.e all days), 8.30am to 8.30 pm, se essentially the same restrictions both sides. This begs the question of what does this sign say now in 2025 ? Has it been changed to Mon-Sat like you said ?
Anyway, at the moment I see no credible appeal option, (at London Tribunals). The council will refuse all your reps informal and formal stating that the PCN was correctly served. On the other hand there may be a possible 'technical' appeal based on Westminster mismanagement of the enforcement process, so before you pay the discount, wait a bit to see what turns up. Some councils have made major
****-ups with their webpages and appeals have been won on this.