Hilarious. You are in the evidential photos. Ask why didn't the bounty hunter taking the photos warn you that you were breaching the terms of parking?
Also, as you point out, what yellow lines? What do the term on the signs in minuscule writing actually say? A close up of the sign would be useful.
No appeal is going to work and this is most likely to be concluded in the ultimate dispute resolution service, the county court. However, there is a greater than 99% chance that it will never get to a hearing and will be discontinued. You will just have to weather the useless debt collector letters (which you can safely ignore) in the meantime.