First, forget the date 1817. That may have been the date of the somewhat inaccurate picture of Longwood used on the vessel but not the vessel itself.
The writing on the bottom of the piece is much more interesting, in referring to the illustration as the Residence of Napoleon Bonaparte and not Napoleon Buonaparte – the original spelling of his name used by the English to belittle the emperor and remind him and the world of his Italian roots tends to date this pot somewhat later than 1821.
Also the exaggerated flourishes used on the N in Napoleon is a much more Second Empire style.
So I would look for a factory producing wares at the time of the return of the body of the emperor to France in 1840 and on into the Second Empire.
Isle of Wight