The British colony was established only though the expulsion of the Argentinian colony. It does not matter how long ago this happened - as the legal maxim goes ‘title does not pass with theft’.  Colonists do not have a right to self-determination. It would be absurd if a group of people could invade some land, drive off the people living there; and then state that they have acquired the right to decide for themselves to stay there. The natural consequence of that principle would be that anyone could gain property through ethnic cleansing and long enough adverse possession.