The route from Deal or Dover to the wood would involve crossing or leaving the motorway at Junction 10 or 10A. Much construction work being carried out at all hours at 10A (constructing new border checkpoints for freight) so someone in the know might choose 10. There are definitely ANPR cameras on the exit roundabout and I believe the next roundabout, known as The Cow, also has cameras on it.
The area here is pretty built up, it only opens out and becomes country at Great Chart. So the vehicle may have been picked up at an odd hour on two sets of cameras on the route to Great Chart.
Possibly also by CCTV at a storage facility further along the road towards the actual site.