You have until Saturday 27th July to submit your POPLA appeal so do not rush it.
They ignored that fact that the NtK is not fully compliant with the requirements of PoFA in their appeal response. Also, the photo of the sign, whilst close up, shows that the £100 charge notification is tiny when compared to the rest of the text in the sign. It does not give adequate notice to bring to the attention of the driver the charge sum.
PoFA 2(2) and 2(3)(b)(ii) explain what is required and meant by "adequate". Compare the sign that gave rise to the Beavis case and the ECP sign. Note the difference in how the charge is "adequately" brought to the attention of the driver:

You should have look at the BPA CoP and research their requirements on signs. ECP always have difficulty defending their rubbish signs:
BPA CoPYour POLPA appeal should be based on, but not limited to, the following points:
(1) Failure to fully comply with the requirements of Schedule 4 of the Protection of Freedoms Act 2012 (PoFA)
(2) The operator has not shown that the individual who it is pursuing is in fact the driver who may have been potentially liable for the charge
(3) Inadequate signage leading to failure to adhere to PoFA 2012 and breach of the BPA Code of Practice
(4) No evidence of landholder authority
(5) Failure to comply with the BPA CoP General Principles for ANPR
(6) Failure to comply with the BPA CoP by not displaying the Approved Operator Scheme (AOS) logo on the signs.
(5) and (6) are only if there is any failure on those points although it does not hurt to add them anyway as they have to be rebutted by the operator.