Horizon is the operator who sent the original PCN. Issued 6 April 2025 for a “breach” on 24 February 2025. In England this would be too late to transfer liability. In Scotland it’s not possible to do so anyway. The PCN gave you a limited time to appeal after 6 April 2025 which has long passed.
You could have replied to Horizon when you received the PCN in 2025. Did you receive it and ignore it, or did you not receive it? In any case, we are where we are.
DCBL has no power whatsoever, and uses red ink to frighten you into paying.
Ignore DCBL but not anything from DCB Legal in future. They will likely bluster and threaten and use red ink but they have no legal basis for pursuing you, the registered keeper, if the driver has not been identified.