Hi, I'm glad you were able to solve the puzzle of where John was between conviction and transportation.
There were a large number of Prison hulks moored in the Solent, off Portsmouth. They were used not only for prisoners awaiting transportation but also for 'ordinary prisoners' ( as more crimes were punished by imprisonment, the old gaols became overcrowded. )They were also used for prisoners of war.
This picture of the hulks off Portsmouth ( and the other 2 similar ones) is thought to have been painted by a French prisoner of war http://collections.rmg.co.uk/collections/objects/13403.html
A blogpost, written by doctoral candidate who is engaged in researching their use is an interesting read