The Portuguese Guitar is a direct descendent of the cittern, WAV. As is the German Waldzither and the Corsican Cetara. The ciittern's development is well documented and here it is simplified direct from Wiki - "it is generally accepted that it is descended from the Medieval citole (or cytole). Its flat-back design was simpler and cheaper to construct than the lute. It was also easier to play, smaller, less delicate and more portable. Played by people of all social classes, the cittern was a premier instrument of casual music-making much as is the guitar today."

Furthermore, if you look into the citole you find "Although it was largely out of use by the late 14th century, the Italians "re-introduced it in modified form" in the 16th century as the cetra (cittern in English), and it may have influenced the development of the guitar as well."

So your theory of it being an English instrument and should be the only one played here is, as ever, bollocks. The same instruments develop and evolve the world over and there are no boundries to music so please stop trying to impose them.