Driver thought signage may not apply for the right turn (rather than full u-turn).
If the aforementioned known issue has been corrected, then driver would not appeal on that basis.
I have appealed using MrMustard's old advice on another (similar) thread but do not expect a favourable response: https://www.ftla.uk/civil-penalty-charge-notices-(councils-tfl-and-so-on)/penalty-charge-notice-50j-prohibited-right-turn-bexley-council-upper-wickham-lan/msg95592/#msg95592