Have been to Rio many times and always take a mixture of Reals and USD. Have never had any problems with USD whereas any spare Reals are not so easily exchanged back to GBP.

You can get Brazil Real R$ in the UK. Not all banks, post offices stock it... But it will only take 1 or 2 days to order it in for you. Without Brazil Real you won't get anywhere in Brazil. So to the banks on Monday, perhaps the Post Office but have my doubts. A word of warning the exchange rates in Brazil are expensive according to my colleague who visits there regularly.