If you get a speeding ticket through the post, which you really disagree with, you are able to request details of the camera used to capture you and its calibration certificate.
You are also entitled to request the photograph and video (if a video exists). You can do so before pleading not guilty, though they do not have to give it to you before then.
In that case you will have to turn down any offer of a fixed penalty.
You should accept a fixed penalty if you feel that you have committed the offence, but not if you feel not. I appreciate that the reality of the situation may be difficult, but if you turn down a fixed penalty then you will be looking at a court appearance.
If you were convicted later then you will be looking at around six points and a fine around the figure of a week's salary.