Im pretty sure I wish I could find more history about it that the the toponym River Plate is a relative neologism coined in the British Empire esp in England perhaps as little as a century or so ago   Youre surety is misplaced The British have been calling it River Plate since Drake With the barest minimum of research I counter your assertion with the fact that a book entitled A narrative of the operations of a small British force under the command of BrigadierGeneral Sir Samuel Auchmuty employed in the reduction of Monte Video on the River Plate AD  By a field officer on the staff Illustrated with a plan of the operations  was published in  See Samuel Auchmuty