Have you been provided with a photo showing the gantry? Does that confirm that the only limit showing was above the hard shoulder?
On a more general point, you cannot "challenge" the offer of a fixed penalty. You can either accept it, or go to court. If you were to lose at court, the cost would be nearer £1,000 than £100, plus 4 to 6 points rather than the three now on offer.
Having no previous offences is utterly irrelevant.