Upvote:1
The broadcast of the 14th May gives this precision "if not already volunteered or requisitioned" because there was other motive to collect such boats before the war. They were used for low waters missions in the Channel, for example:
So the broadcast should not disturb theses boats from their mission. The difficulty to follow the war and the necessity to keep secret the point of Sheerness at least at the beginning meant that a local owner of boat had no idea of where to lead his ships for volunteer.
However, he could have asked at his local city and get this information.