You can take US dollars if you wish. Of course that would involve purchasing them in the UK and getting hit with an exchange fee and then getting hit again each time you convert the US dollars into pesos and reais. So double the fees. THE USD isn't legal tender in any of the countries you intend to visit.
Or you could just do what the rest of the world does and take out local curencies from an ATM witha debit or credit card.
Get money out at the ATMs, before you go to Argentina I'd suggest getting out USD. The official exchange rate is 4.90 pesos to the dollar, but the blue rate is 6.70 so it's almost a 30% difference.
Assuming you're taking a plane to Argentina the airport should have an ATM with the option to get USD out.
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