Britain has good relations with Argentina anyways (they were an honorary Commonwealth member in practice in the early 20th after all)
The success of the revolution in Argentina probably affected that, tho. The relationship between Britain and Argentina was indeed very good, but it was based on a quasi-neocolonial model, with Britain all but controlling the economy. They made up large part of the exports, and had a near monopoly on investment, primarily in infrastructure for export and export related industries.
This state of affairs was held up in Argentina mainly by the landowner elites that dominated the country around that time, and while it led to some of the most prosperous times of the country, it wasn’t adaptable and made Argentina overtly dependent on Britain, as well as stunted industrial growth. Indeed, when the Great Depression came and Britain closed off the empire to imports, Argentina was supremely fucked, and had to scramble to sign an
extremely lopsided treaty to get a market from the exports. This was, btw, the moment in which many of the problems that plagued Argentina for the rest of the century started.
Anyways, enough context and back to ITTL, Alem and Co. would’ve likely aimed for a different model that would’ve come into conflict with British dominance, favoring industrialization over export-oriented economy. The alignment of Argentina with America would also play a part, since it would’ve likely also involved an introduction of American investment to compete with Britain’s. Even more post-war, I’d say, since USA would probably be sure to keep their only real ally, from both an ideological and political perspective, strong and in their camp.
So by the time of the war, while Britain would likely still enjoy a fairly good relationship with Argentina and would still take a substantial amount of their exports, the picture that London would probably get is of a country slipping further and further away from their control, and not someone they can freely trust.