Having always been a fan of Perceval I feel his death robbed the UK of a great prime minister.
He favoured supporting the army with cash, rather than retoric, so the moral in the army would have been that much higher, and I like to think this would have made Wellington's victories more significant, so he would enter onto French terrotory earlier.
With regards to America I don't think that it would have changed anything, although as a friend of the Duke of York I think he would have pushed for the changes to the army to have been faster / more far reaching, so the Brit's may not have had a disaster at New Orleans.
Above anything else he was a humanitarian and was had a top quality financial brain (as can been seen from his work as Chancellor). I like to think that he would have found a way to defuse the situation that lead to the Peterloo massacre.