I wouldn't miss the 23rd December deadline because I think the fine can go up from £100 if you take too long. Are you able to give them the information they need?
The GDPR breach is separate thing, it sounds serious. Hopefully other people will be along to advise on that.
Their idea that they will take their £125 reduction letter to prevent you claiming costs is laughable. They have so many things wrong with their claim that they ha"ven't a hope of getting £1 far less £125.
Going back to your post 45, you only showed two of their notices, neither of which are apparently their T&Cs according to the notice numbered 5. It also doesn't say that not paying will result in extra charges being levied. It does say it on picture 2 but that is not their T&C notice. That notice on 2 is confusingly worded. "Anyone parking in a blue bay and leaving/walking off site will result in an immediate issue of a parking charge notice." Does that mean it is only those parking in disabled spots that cannot leave the site. Or does it mean that anyone parking in a blue bay, and anyone going off site is liable to get a PCN?
I sent in a Self Assessment for Self Employment by the 31 October 2019 deadline and the Inland Revenue has written back to me asking for some more information on the supplementary evidence page. They say they have sent with this a scanned image of my tax return but they have sent me someone else's SA102 form. This has their name, address NINO, full bank account details and all their income information. My worry is where has my information been sent to? This is surely a breach of data protection. Any advice please on what I should do? They say they are going to fine me £100 if I don't get my reply back by 23 December 2019.