Out of interest, is anyone aware of any case history where someone has had a speeding charge overturned due to dash cam footage? The reason I ask is that speed cameras have to be calibrated regularly and are extremely accurate. Dash cam footage would not be as credible as a defence against them as they aren’t subject to the same calibration requirements etc.
As others have said, they have usually clocked your speed before you see them, so the speed you were going when you saw them isn’t always the speed that they clocked you at, which is often why people think that they were going slower than the fine suggests.
Dash cams are more useful in accidents, but a word of caution is that having a dash cam can mean that your dash cam footage can ultimately prove that you were in the wrong, rather than the other person! It doesn’t always follow that the person with the dash cam is blameless!