Sept 29-30
Logistics is the problem right now. The Army will operate at reduced capability beyond 3 MPs from a supply depot. Supply depots can be chained, so doing that would extend the operational area to the London approaches.
The rail lines out of Dover and Brighton are currently damaged, so it is necessary to bring across a rail repair crew from France. Putting those crews onto the limited transports will cause a many days delay in bringing across a Panzer regiment or an Infantry brigade.
So over the next 4-6 days, the currently available supply depots will have to be chained from either Brighton or Dover, and rail repair crews will have to be transported across the Channel.