I have never seen one of these plates before now.
I believe it to be a very early transfer printed pearlware octagonal plate, probably 19th Century. The centre piece is Nelsons tomb being hugged by a mourner in a square/star cartouche, the rest of the plate depicts seashells and coral.
There is an age related crack running from 1 o'clock, down and over to 3/4 o'clock, not particularly visible from the front but much more so from the reverse.