Many of the “locals” were unsure who to side with your representing this as a black and white situation. Britain also considered making Buenos Aires a proctectorate which whilst alters the perimeters of the original thread was equally as plausible as a “colony”. Britain had held long term ambitions to gain access and to dominate trade in South America. A plan of political concessions and appeasement’s were allegedly being drafted to help acquiesce the population into the British sphere of influence post invasion. Whether these were to be successful is highly debatable but I believe several outcomes could have been possible in an altered time line rather than the stock response which may indeed have been the result but seems to be a tired narrative without due consideration to other potential narratives.
Tired narrative? The tired narrative is that placing Anglo-Saxons in the Rio de la Plata would magically turn Argentina into a first-world country, never mind that it was doing just fine until well into the twentieth century without Anglo-Saxons. You are severely underestimating the loyalty of the Argentina people to their king, and you are underestimating the strength of Spain (whose fatal decline has yet to occur).
I also doubt that the British would be successful in bringing in local populations to their side. In Spanish America, the upper classes were uniformly in support of the Spanish monarchy, until that monarchy was overthrown by Napoleon and Spanish political theory meant that in the absence of a king, authority reverted to the people. There were rebellions such as the Revolt of the Comuneros, but that was an extremely minor affair, and strikingly, those rebellions were in the name of the Spanish king. No doubt protectorate status would have been less awful to the Argentinians, but it would have been insufficient to make them support the invaders, unless that protectorate status means little more than trading with Britain, in which case we essentially have OTL Argentina.
But, sure. Here’s a scenario. Britain invaded and takes Buenos Aires in 1808. Despite initial resistance, word of Napoleon’s invasion of Spain leads to ambivalence by the local population, who are happy after Britain gives them protectorate status. Yet, confederal Argentina, after it gains independence, is angry and fights numerous wars against Britain.
Would it be possible for the British to "Plantation" the River Plate somehow to insure their loyalty? Perhaps by using the folks that IOTL would be sent to Australia?
Some of the people placed in Australia were Irish political prisoners. That would, if anything, lead to sympathies among Argentinians, many of whom are descended from Irish people who fled Ireland.