Don't pay.
If you give into their threats, thinking you can appeal later, forget it.
Am I right in assuming that you are the registered keeper? If so, and you weren't the driver, simply write to them saying you don't know who was driving at the time and the onus is on them to provide evidence of who was driving.
Even if they give a description of the driver from CCTV footage, you are under NO legal obligation to reveal the persons identity to them.
I must add that if YOU were the driver and are caught on CCTV, don't lie about it, but then revert back to your defence of the signs being obscured.