Thanks all for your input, I come back to check and wasn't expecting a spirited discussion.
I ran a diagnostic of the GPS of the dashcam, and it does show that when I reached the 30 sign post, my speed was below 30, so that's the fact, no more assumption/presumption.
As mentioned near the top of page 2, I will leave this to judge. I will provide an update once the dispute is over, cherers