In Aug 1822 Lord Castlereagh slit his own throat with a penknife. What if he did not and his depression lifted enough for him to remain functional? He was much more favorable to the Congress System and the Holy Alliance than Canning.
One particularly interesting aspect of this is that Castlereagh may not have opposed the Holy Alliance's desires to send an expedition to the Americas to put down the rebellion in Spain's colonies.
Tom
I think if Castlereagh lived there would probably be a more right wing government as I think he was also a rival of Canning. However your still got the economic question. Britain was making good money trading with the freed colonies and knew it would lose that trade if Spain regained control. [Unless possibly the French and Spanish made some deal with Britain possibly]. Even then powerful elements in Britain will probably not be too willing to see France gain too much power.
If some deal was reached and Britain stood aside then things could go very bad for the rebels. France has a lot of Napoleonic veterans who the monarchy would probably be glad to get well away from France and they could do a lot of damage to the rebel forces. With some of the Marshals leading them as well even with the vast differences involved. They also have plenty of experience of fighting with poor logistics and living off the land. [Although if conflict lasted a while you might see defections, possibly in considerable numbers, which could add an interesting complication]. It could be a lasting quagmire for the French and Spanish however and a later change of mind by a different British government, say after some massacres, could possibly force them out. How that would affect the development of Latin America would depend on the circumstances but a lot of the initial leaders OTL could be lost.
A stronger right wing element in Britain could led to further repression but also a stronger reaction when it occurs. Hopefully change will still come and be largely peaceful but might not be and could be stronger for being delayed.
You are highly unlikely to see a Monroe Doctrine in TTL, or, if he is rash enough to make it, it will be seen as a dead letter as the US doesn't have anything like the strength to intervene at this time period. Again either of those options could have effects further down the line. Possibly an even more isolationist US taking no interest in events outside its borders?
Steve