I think their fear was Airborne invasion/assault on airfields at that stage more than how quickly the tanks got to them. On the basis of what happened in Crete the following year maybe not so fanciful............
Actually I am reminded of the words of William Woollard in the 'Secret War' [BBC] when he was investigating the Gigant Glider - That at this moment in time the Me321 Gigant must seem complete folly, but at that time [in 1940] 'half a dozen of them descending out of a dawn morning into a field in Southern England with little more than the rifles of the Home Guard to stop them..............' shows just how close run a thing it may have been..........
I live in Norfolk !!, frankly between the North Norfolk Coast and Colchester there wasn't a great deal to stop an invasion, there are many fortifications of that era dotted about, to say they seem inadequate in the understatement of the year...............
Arc