If they decide to decline the appeal, means I will have to pay for it?
It's not an appeal, it's just an informal representation. Much as TFL would like to be judge, jury and executioner, it's not actually up to them whether you have to pay because we don't live in North Korea. It's only if you lose the appeal to London Tribunals that you actually have to pay.
You should probably read this page to understand the full process:
This section explains the enforcement process for a parking Penalty Charge Notice, where the vehicle was not clamped or removed.
Details of the appeal process itself are provided here
This page is designed to help you understand when you may appeal to the adjudicator and what steps you need to take to do so. When can I appeal? You may only appeal to the adjudicator if: You have...
but note that where they mention a "
Notice of Rejection" that refers to a formal Notice of Rejection that is only issued in response to formal representations made against a Notice to Owner; informal representations against a roadside PCN only lead to an informal rejection which cannot be used to appeal to the tribunal.
After the informal rejection has been received, the next step in the process will be a Notice to Owner and we'll help you with the formal representations.