OP, I agree: where is the evidence of a contravention?
All the photos show is you stopped adjacent to dashed white lines - so what?
If the CEO wants to rely upon what I believe is TfL's position which is that you should have known that white markings on a length of road between RR restrictions indicate the full time presence of and conditional exemption from a red route restriction, as per the fore and aft DRL, then it's obvious why they didn't include the traffic sign because it's part-time!
IMO, the rules relating to red route markings are unclear.
However, I think you could put the following to TfL.
You parked adjacent to white road markings as shown in the photos. As these were not red, you interpreted their meaning as being lengths of road excluded from the red route 24/7 prohibition as indicated by the fore and aft double red lines and parked accordingly. After being served with the PCN you researched the matter and discovered in TfL's own publications that actually these markings indicate areas where parking or a similar activity is permitted for the whole of the time of the prevailing red route prohibition. However, in this case you have discovered that there's also a traffic sign which indicates a part-time exemption.
You submit that either TfL use white markings only for full time exemptions(from the prevailing prohibition) or they should abandon these and use red markings for all areas instead of, what is the case here, which is a part-time exemption to a full-time prohibition. The use of white markings for either full or part-time exemptions cannot be anything other than confusing.
IMO, the signage is improper.
OP, the permitted markings for a Red Route are:
SRL;
DRL;
Dashed red line, signifying a 'parking' bay which may be used only for part of the time that the RR is in force;
Dashed white line, signifying 'parking' bay in use for ALL the RR hours(although the convoluted wording of the Traffic Signs regs suggests otherwise).
These markings are white and the RR is 24/7 as indicated by the DRL, therefore IMO you may not have a part-time exemption, as here.