I had a look on GSV, but its latest view is June 2022, and this bay is not there, so it is clearly a later addition. There is a sign by the bay, which will give the bay conditions, but it can't be read on your photo. Did your father look at the sign ? The words on it should state that it is for BB holders, who also have a permit of the code shown on the carriageway text. If the sign and the carriageway text contradict each other, he has a strong case.
We always recommend that PCN recipients submit representations against a PCN, even though it may seem they are bang-to-rights from the presented evidence. As a BB holder, you should be dealt with more sympathetically. HOwever, before you submit reps, would it be possible for you to take photos of the bay, in particular the bay sign, as this will influence how you format your representations. Also wait for others to comment. If reps are received by the council within the discount period, they will re-offer the discount if they reject those reps.