The speed that you were doing at the point you passed the van is utterly irrelevant in itself.
The speed that you were doing at the point the device in the van recorded your speed as 80mph is very relevant.
If the device has line of sight to your car, it could potentially record your speed at up to 1km away. At 70mph, it would take ~32s to travel 1km - so, did you speed get anywhere near 80mph within ~32s of passing the van (assuming that your speed was reasonably constant)?
N.B. The offence is exceeding the 70mph speed limit, not specifically doing 80mph - if you can prove that you were only doing say 77, you almost certainly would still be convicted.