My Daughter has just got back, from Argentina. She has Dual UK and Aus passports. She also speaks Spanish quite well. When she was entering Argentina, she pulled both passports out. The official was very suspicious, and she was asked a number of questions, in Spanish and English. She was allowed entry, but it was after an hour and a half. Later a Spanish friend (from the Spanish consulate)told her that the Argentinians do not like the British and the dual passports would have made them think that she was up to no good.
If you are only going to Argentina and Brazil, the UK passport is not needed. Also if you are entering UK through Newcastle, Manchester, Glasgow and you are staying for only a limited time, it is quicker to use the Aus passport as they are usually not very busy.
PS My wife and I both have UK and Aus passport, and have also travelled in South America and go back to Scotland twice a year. So I know what I'm talking about.