There's quite a lot of motorway that isn't 'smart' or planned to be so, and quite a lot of other possible road that could be used - it may not even have to be dual-carriageway, depending on the aircraft. But it's not as easy as 'let's land there' in a once-off trial. For practical use you'd need to prepare and build in facilities on the ground, as was done in Germany, so as to be able to adapt to sustained operation.
Does anyone know whether any bits of road in the UK have been designated as potential sites? As a casual driver-around-the-UK, not many roads seem to me to be straight enough or flat enough!