I m 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 You re 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 Brigadier General Sir Samuel Auchmuty employed in the reduction of Monte Video on the River Plate A D 1807 By a field officer on the staff Illustrated with a plan of the operations was published in 1807 See Samuel Auchmuty